The present epidemiological study concerned and evaluation of the level of measles antibodies (hemagglutination inhibition (HI) assay) and para-influenza-1 (Sendai) antibodies (complement fixation (CF) test) in serum of 107 control individuals (38 women), 176 multiple sclerosis (MS) patients (93 women), 717 relatives to MS patients (361 women), 9 patients with systemic lupus erythematosus (SLE) (all women), 46 relatives to SLE patients (28 women), 57 patients with rheumatoid arthritis (RA) (37 women), and 143 relatives to RA patients (85 women). In MS and their relatives the HI titer value was significantly raised and the CF titer only insignificantly increased. In SLE the HI titers were insignificantly raised but the CF values significantly decreased. In RA HI values were insignificatly raised, but the CF values were significantly decreased among females lacking rheumatoid factor in serum. In the individuals under study, HI values did not correlate with CF values. In MS two groups of patients could be treated, i.e. one group with raised HI values and one with normal distribution of titers. The data obtained are discussed in light of the theory, that all three disease entities may be "Slow Virus Diseases".