30 patients with systemic lupus erythematosus (SLE), 19 patients with rheumatoid arthritis (RA) and 34 controls were investigated with regard to measles antibodies (AB). The titres were compared with clinical and immunological parameters. As previously reported by several authors we found increased measles AB titres more frequently (kappa 2-test, p less than 0.05) in patients with SLE than in those with RA and controls. No significant correlation was found between immunoglobulins, antinuclear antibodies, CH50 or rheumatoid factor, and an elevation of the measles AB titre. A trend was noted, however, towards a positive correlation between measles antibodies and IgM. An elevated measles AB titre in SLE can be explained on the basis of several mechanisms. However, hyperimmunoglobulinaemia with non-specific, secondary AB elevation can be ruled out as a cause of elevated measles AB titre according to our results.