The role played by viruses in cancer is reviewed, with reference both to those viruses known to cause cancer and to those viruses which are associated either with tumour viruses or with tumours, but do not apparently cause tumours. The isolation and properties of one of these tumour-associated viruses, Kilham rat virus, is described. For large scale production of virus RV was grown in primary rat embryo tissue culture. It was found that RV could also be propagated in a variety of rat cell strains, and in a hamster cell line, but not in mouse embryo cells. The virus was purified from infected cultures by enzymic and detergent treatment of disrupted cells, followed by centrifugation in a caesium chloride density gradient. A plaque assay was developed for RV in secondary rat embryo tissue culture. The plaque production was proportional to RV dose up to thirty plaques per plate, and was inhibited by RV antiserum prepared in rabbits. The production of high titre infectious RV and the development of a plaque assay enabled the growth of RV to be studied under one cycle conditions. The growth curve showed that mature infectious RV was produced by about 6 hours after infection, but that the exponential growth was slow and prolonged, continuing for about 24 hours. If RV antiserum was included in the growth medium, the number of infected cells was lowered, and the exponential growth period was shortened. This suggested some asynchrony of infection. Studies on cell fractionation and with fluorescent antiserum showed that during the growth of RV, viral antigen accumulated almost entirely in the nucleus. Analysis of RV by caesium chloride density gradient centrifugation showed that haemagglutinating activity of the virus was associated with 20 mum particles which banded at buoyant densities of 1.42 g./ml. and 1.34 g./ml. Analysis of the gradient fractions by plaque assay showed that the dense particles were infectious, but that the light particles were not. Dense and light particles had sedimentation coefficients of 115 S and 73 S respectively. It was estimated that the dense RV particles had a molecular weight of 6 x 10e6 daltons. Electron microscope studies showed that the dense and light particles appeared "full" and "empty" respectively. The appearance of the particles was consistent with an icosahedral structure composed of 32 hollow capsomeres. Studies on the incorporation of [3H]-thymidine and [3H] showed that the dense RV particles contained nucleic acid, and that the nucleic acid was DNA, Analytical ultracentrifuge studies showed that the DNA was single stranded, a finding which was confirmed by the ability of formaldehyde to react with the bases of the nucleic acid in intact particles. Band sedimentation of the virus DNA through 99 caesium chloride solutions indicated that the molecular weight of the DNA was 1.7 X 10e6 daltons. The biological and physico-chemical properties of RV are discussed and compared with those of other small DNA viruses. A preliminary classification has been suggested for RV and other small DNA animal viruses with cubic symmetry. One group contains the adeno-associated viruses which are dependent on another virus for replication, and possibly have a double-stranded nucleic acid. The other group contains the H-1 and H-3 viruses, minute virus of mice, and Kilham RV, which replicate independently, and contain a single stranded nucleic acid.