Serum samples which had previously been found positive for LAV/HTLV-III antibodies by the ELISA test and then confirmed by radioimmunoassay (Western blot) were tested for the presence of neutralizing antibodies. No neutralizing activity was found in the sera of a group of patients with the clinical diagnosis of AIDS. However in patients with LAS and other related pathologic conditions the percentage of sera positive for neutralizing antibodies was 27% and 55% respectively. At least 50% of the sera from seropositive asymptomatic individuals had neutralizing activity but with the exception of the haemophiliac group the mean titre was much lower than that of LAS patients. No relationship was found between the neutralizing titre and the antibody specificity detected by Western blot analysis for p41 and p120
