The aim of this thesis was to investigate a broad variety of factors, in a cohort of EU heterosexual partners of HIV⁺ individuals (indexes), which may affect heterosexual transmission. The immune function of the EUs was assessed and compared to normal controls, by monitoring proliferative responses to mitogen, recall and alloantigens and a combination of recombinant HIV proteins. Cytokine responses to these stimuli were also monitored. The EUs were also confirmed to be uninfected by polymerase chain reaction (PCR). The EUs had similar proliferative responses to controls for both the allogenic and recall antigens and showed a minor difference in the response to the mitogen, phytohaemagglutinin (PHA), which may reflect differences in the kinetics of the response. An increase in the amount of interferon-γ (IFN-γ) produced in response to alloantigen was seen in EUs compared to controls, which could potentially inhibit HIV replication. The proportion of lymphocytes expressing the MHC Class II protein, human leucocyte antigen-DR (HLA-DR), was also elevated in the EUs compared to controls and may reflect an overall increase in the activation status of the EU' lymphocytes. Genetic factors which were investigated included the HLA antigens and the recently reported mutations in the CC chemokine receptors (CCR), CCR-2 and CCR-5, utilised by certain strains of HIV as co-receptors for entry. The HLA allele DR5 was elevated in frequency in the EU cohort compared to population controls and to HIV⁺ individuals who were infected by heterosexual exposure.