A possible association between the endogenous retrovirus, ERV3, and multiple sclerosis (MS) was exmined. Samples of DNA from 74 MS patients and 159 healthy blood donors were subjected to enzymatic amplification followed by single strand conformational analysis to detect polymorphisms in the long terminal repeats of ERV3. Using this approach we detected six single base pair variations and a drop-out of a nucleotide. The linkage pattern of these base pair variations enabled us to define three allelic forms of ERV3. Polymorphisms exclusively present in the group of patients were not found and the distribution of the three allelic forms did not differ significantly between the group of controls and the MS group. Neither was there a significant difference in the distribution of the three alleles between MS patients with the progressive form and patients with relapsing/remitting MS. Our results are not in support of an association between ERV3 and MS.