Our study focused on the relationship between lymphocyte H/S ratios and antiviral antibodies. We have concluded a longitudinal survey of 29 clinically definite MS patients for 8 months seeking a possible correlation among clinical modifications, fluctuations of T-cell subsets and antiviral antibody titers. For this purpose in each patient the following parameters were recorded monthly:--clinical history and neurological examination;--blood T-cell subpopulation as defined by monoclonal antibodies of the OKT series;--serum antibody titers against measles, herpes simplex type 1 and 2, HTLV-1 viruses. All these data were compared with the different MS subgroups as defined by clinical course: stable, relapsing, progressive. We found that constantly normal OKT4+/OKT8+ ratios correlate with a stable clinical course; that highly fluctuating ratios parallel clinical exacerbation; that constantly high ratios are associated with a chronic-progressive course. These results are discussed with special reference to the correlation between OKT8+ lymphocyte percentages and anti-HTLV-1 antibodies, since this last virus has recently been claimed to be of pathogenetic importance in MS.