Antimicrobial peptides form an important aspect of the innate immune response of mammals. They function to kill invading microorganisms and in many cases activate other aspects of the innate and adaptive immune systems. One of the largest families of antimicrobial peptides is the defensins. In vertebrates, there are three subfamilies of defensins, termed α, β and θ based on the connectivity of the six conserved cysteines. The β-defensins, which are produced mainly by epithelial tissues and keratinocytes, came to wider interest when it was proposed that their loss of function might play a role in the pathogenesis of cystic fibrosis (CF) lung disease. Previous work has suggested that the airway surface liquid (ASL) of primary cultures of human airway epithelial cells possess salt-sensitive antibacterial activity and that this is impaired in CF individuals by elevated levels of sodium chloride. Further work has also suggested that the β-defensins secreted by the airway epithelia might comprise an important component of this salt-sensitive defence system. The aim of this project was 1) to characterise the salt-sensitive antibacterial activity of members of the human and murine β-defensin subfamily, 2) to analyse their activity as chemoattractants, 3) to establish a cell culture-based system for the production of β-defensins to allow for greater analysis of their range of activities and 4) to verify the validity of novel human β-defensins identified by bioinformatics techniques. In this thesis describes the characterisation of the salt-sensitive activities of synthetic human β-defensin 2 (DEFB2), mouse β-defensin2 (Defb2), and a novel β-defensin related 1, Defrl, which lacks the first of the canonical six cysteines are described against a range of CF-related pathogens. This work has concluded that a) DEFB2, Defb2 and Defrl display, to varying degrees, salt-sensitive antibacterial activity, b) The differences observed between the activities of the peptides may represent the evolution of species-specific profiles of antibacterial activity for specific defensins. c) That the loss of in Defrl of the first canonical cysteine does not result in loss of antibacterial activity and, most interestingly, Defrl also demonstrates activity against B. cenocepacia - a pathogen normally resistant to the activity of antimicrobials. Data presented in this thesis also suggests that synthetic Defb2 and Defrl show chemotactic activity to CD4+ T-lymphocytes and to immature dendritic cells. This work concludes that, like human β-defensins 1 and 2, the murine β-defensins, Defb2 and Defrl, can act as a bridge between the innate and adaptive immune systems. In this thesis, expression pattern of five novel human β-defensins in a range of human tissues is also analysed. Evidence is presented that they are all expressed at high levels in the testis and that two of these genes are expressed at much lower levels a variety of other tissues. These data suggest that the β-defensins are an expanding, and potentially quite large, subfamily of genes, many of which are yet to be characterised in terms of their expression profile and the antimicrobial and chemotactic activities.