The work presented in this thesis describes the isolation of a series of temperature-sensitive (1s) mutants of herpes simplex virus type 1 (HSV-1), Glasgow strain 17 pia, and their genetic and bio-chemical analyses. Eighteen ts mutants were isolated from virus stocks treated with 2-aminopurine, acriflavin, proflavin and ultraviolet (UV) light. Two ts mutants were isolated after treatment of purified intact HSV-1 DNA with hydroxylamine. Of these twenty ts mutants, fifteen were studied and ranged from genetically non-leaky to very leaky. Only one ts mutant studied appeared to have a less efficient one-step growth curve that the wild type (wt) virus at 31°C. None of the ts mutants appeared to be more thermolabile than the wt virus in heat inactivation experiments. The fifteen ts mutants studied fall into twelve complementation groups; three was the largest number of ts mutants in any one group. Furthermore, four of the newly isolated ts mutants fall into complementation groups represented by previously isolated ts mutants. Eight of the ts mutants synthesise undetectable amounts of virus DNA at 39°C, while the remaining seven synthesise virus 1MA at this temperature. All the ts mutants studied synthesise normal amounts of viral DNA at 31°C. All the ts mutants studied synthesice wt amounts of the herpesvirus-induced enzymes thymidine kinase, UV. polymerase and DNA exonuclease at 39°C and 31°C, although two ts mutants induce thymidine kinase activities that are more thermolabile than that of the wt virus. The thermolability of the thymidine kinase activities induced by these ts mutants is consistent with the finding that these two ts mutants both synthesise altered 43,000 mole-cular weight (MV) polypeptides at 31°C and 39°C; the 43,000 M poly-peptide is thought to be a monomer of the active enzyme possessing thymidine kinase activity. Attempts to complement the defective thymidine kinase activities' coded for by these ts mutants in vitro and in vivo with the wt virus were unsuccessful. Analyses of the polypeptides synthesised by the ts mutants showed that a number of polypeptides induced by several of the ts mutants are altered in their electrophoretic mobility at 31°C. The presence of some of these polypeptide defects does not appear to significantly inhibit the virus lytic cycle at 31°C. Analysis of the ts mutant-induced virus polypeptides synthesised at 31°C and 39°C indicates that none of the fifteen ts mutants studied have identical infected cell polypeptide profiles at 39°C. This is consistent with their clonally distinct isolation. Three mutants which fall into one complementation group have similar but not identical infected cell polypeptide profiles at 39°C; a situation described earlier for three ts mutants - ts T, L and Kan. The ts mutants were also analysed for their synthesis of virus glycoproteins and phosphoproteins at 31°C and 39°C. Analysis of their glycoprotein synthesis showed that three mutants are defective in the synthesis of the gC glycoprotein, a glycoprotein thought to be involved in the cell fusion process. Analysis of the phosphoprotein profiles of the ts mutants at 31°C showed that several ts mutants code for temperature-independent mutations which result in the alter electrophoretic mobility of one or more of their phosphoproteins. The most commonly affected phosphoprotein was the P65'(65) polypeptidr, which was reduced in labelling intensity, absent, or exhibited an electrophoretic mobility change in five out of fifteen ts mutants. Investigation into the synthesis of phosphoproteins by the ts mutants at 39°C showed that all but one ts mutant exhibit a more restricted phosphorylation profile than that of the wt virus at 31°C and 39°C. Tryptic peptide analyses of some of the phosphoproteins induced by the wt virus with different proteases, suggests that the phosphoproteins P65k65) and P58 may be derived from the same gene product. The possibility that in vitro phosphorylation of certain molecules can occur in the presence of partially purified herpes phosphoproteins in the absence of adenosine triphosphate (ATP) is discussed. The synthesis of virus structural antigens at 39°C by the ts mutants, as detected by immunofluorescent staining, was studied. In general, most of the ts mutants expressed a more restricted pattern of fluorescence than the wt virus at 39°C. Moreover, the DNA negative mutants tended to lack specific antigen staining at the cell boundary, while this localisation of antigen was evident in the DNA positive ts mutants at 39°C. These studies reveal that, in general, the ts mutants can be distinguished as early or late ts mutants by their immunofluorescent staining patterns, and that ts mutants in the same complementation group may have similar but not identical patterns of immunofluorescent staining.