Neutrophil, monocyte and lymphocyte chemotaxis was investigated in 19 patients with active rheumatic fever (10 with carditis), in 15 patients with rheumatoid arthritis and in 20 healthy, age-matched controls. Chemotaxis assays were repeated in the rheumatic fever patients on the fifth day of therapy and two weeks after remission. Neutrophil and monocyte chemotaxis was found to be significantly decreased in the rheumatoid arthritis patients when compared with the controls and rheumatic fever patients. In contrast, neutrophil chemotactic activity was significantly higher in the rheumatic fever patients when compared with the healthy controls. Monocyte and lymphocyte chemotaxis in patients with rheumatic fever was not significantly different when compared with the controls. Neutrophil, monocyte and lymphocyte locomotion was found to be significantly decreased on the fifth day of salicylic acid or prednisolone treatment.