Multiple sclerosis (MS) is thought to be an autoimmune disease precipitated in genetically susceptible individuals by environmental factors. Recent attention has focused on the possible involvement of retroviruses in its aetiology. Initial experiments performed to detect the human retrovirus HTLV, in lymphocytes from 12 patients with MS, proved negative. In an attempt to identify a putative novel human retrovirus, a polymerase chain reaction (PCR) technique was developed which was capable of detecting a very diverse range of retroviruses including HIV, HTLV, MPMV and MMLV. This 'Pan-Retrovirus' PCR employed semi-nested, degenerate primers complementary to the two most highly conserved motifs of the pol gene. Using this technique a novel retroviral sequence, designated MSRV c-pol, was detected in the serum from a patient with a 12 year history of MS. This sequence was also present in retroviral particles which had been isolated from MS patient derived tissue cultures in France. MSRV is related to the endogenous retrovirus ERV-9, however it remains uncertain whether MSRV itself is an exogenous or endogenous retrovirus. By combining the 'Pan-Retrovirus' PCR with a hybridisation-based detection assay, MSRV c-pol RNA was detected in serum from 24 of 40 (60%) patients with MS but not in 30 controls; and in cerebrospinal fluid from 5 of 10 patients with MS but not in 10 other neurological disease controls. An MSRV specific RT-PCR assay was also developed. This detected virion-associated MSRV pol RNA in serum from 9 of 17 (53%) patients with clinically active MS (in all 6 of those not undergoing immunosuppressive treatment), compared with only 3 of 44 (7%) controls. A novel human retroviral sequence has been identified, and an association demonstrated between the presence of MSRV-RNA and MS. Further work will be required to determine the significance of MSRV in the aetiopathogenesis of this common neurological disease.