Propionibacterium acnes forms part of the normal flora of the skin, oral cavity, large intestine and the external ear. Historically, P. acnes is considered to be of low virulence; however, in recent years it has been found as the aetiological agent in various pathologies including acne vulgaris, endophthalmitis, endocarditis, osteomyelitis, sarcoidosis, prosthetic hip infections and sciatica. It currently remains unclear why this normally harmless commensal can cause infection and contribute to a number of clinically significant conditions. This thesis has sought to investigate the phenotypic, genetic and antigenic properties of P.acnes strains isolated from sciatica patients undergoing microdiscectomy, normal skin, blood cultures, prosthetic hips and acne lesions. Isolates' phenotype was examined by determining their biotype by analytical profile index, antimicrobial susceptibility, virulence factor expression and serotype. A molecular typing method for P.acnes was developed using random amplification of polymorphic DNA (RAPD). Patient serum was used to screen P.acnes strains for antigens expressed in vivo and the chemical composition determined. The serodiagnostic potential and inflammatory properties of identified antigens were assessed. The optimised and reproducible RAPD protocol classified strains into three major clusters and was found to distinguish between the serotypes I and II for a large number of clinical isolates. Molecular typing by RAPD also enabled the identification of a genotype that did not react with the type I or II monoclonal antibodies and these strains may therefore constitute a previously undiscovered subspecies of P.acnes with a genetic background different from the type I and II serotypes. A major cell associated antigen produced by all strains was identified and characterised. A serological assay based on the antigen was used to measure IgG and IgM levels in serum from patients with acne, sciatica and controls. No difference in levels of antibodies was detected. Inflammatory properties of the antigen were measured by exposing murine macrophage-like cells and measuring the release of nitric oxide and tumour necrosis factor-alpha (TNF-��). Only TNF-�� was elicited in response to the antigen. The phenotypic, genotypic and antigenic properties of this organism may provide a basis for future studies on P.acnes virulence and provide an insight into its mechanisms of pathogenesis.