Human endogenous retroviruses (HERVs) are estimated to form approximately 5% of the human genome. While the majority of sequences are defective, containing premature stop mutations and frameshift mutations, a number encode fully functional proteins. HERVs have been proposed as aetiological agents for a variety of autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus, and multiple sclerosis (MS), and have been detected in a variety of tumours. The study aims to develop tools to detect and investigate human endogenous retroviruses in order to establish their roles in MS and anaplastic astrocytomas. A method of detecting and quantifying levels of HERV-W env messenger ribonucleic acid (mRNA) and MSRV gag by reverse transcriptase polymerase chain (RT-PCR) reaction in a variety of cell lines was developed, with PCR products detected in all cell lines tested, and in particular, high levels of transcription occurring in the BeWo choriocarcinoma cell line. In the astrocytoma cell lines, those with P53 mutation had higher levels of HERV-W env. MSRV gag variants were also detected in these cell lines, but stimulation with interferon-γ, a proinflammatory cytokine, did not alter expression significantly. An antibody against an epitope of MSRV gag has been successfully developed, purified and tested to determine the expression of a predicted linear epitope. This epitope was recognised in all cell lines tested, but unusually for a HERV showed nuclear expression. Further analysis is needed to confirm the identity of the protein detected. Finally a number of retroviral peptides with homology to putative antigens were predicted using a novel bioinformatics approach, of which two, HERV-W env 412 and MSAV gag 274, were tested in an enzyme-linked immunosorbent assay of plasma samples from MS patients, patients with other neurological diseases and normal healthy donors. No significant differences in antibody titres were found between the sample groups for either peptide.