The initial phagocytic rate of IgG coated polyvinyl toluene latex particles was assayed for isolated PMN cells. The kinetic studies of phagocytosis were performed in serum-free medium using an electronic particle counter. The mean phagocytic rate for the reference group of apparently healthy individuals was 0.60 min-1. PMN cells from 5 of 26 patients with rheumatoid arthritis (RA) had subnormal values of particle ingestion rate, although the mean of the group was not significantly reduced compared to the controls. Correlation coefficient between the log of rheumatoid factor titer and phagocytic rate was 0.58 (P less than 0.001). During penicillamine treatment an increased rate of phagocytosis was observed in patients with RA. Granulocytes isolated from 17 patients with systemic lupus erythematosus (SLE) had a significantly increased (P less than 0.001) mean initial rate of phagocytosis (0.91 min-u). Two patients in this group had an acute exacerbation of disease activity and were treated with large doses of prednisone. An appreciable decline in phagocytic rate was observed during the intensive corticosteroid therapy.