The acetylation rate of sulfamethazine was studied in 25 patients with systemic lupus erythematosus (SLE). Seventeen of 25SLE patients, 68%, were slow acetylators compared to the reported incidence of 52% in the general population. There was no correlation between the acetylator phenotype in SLE patients and the various clinical manifestations or the activity of the disease. Lupus patients who were slow acetylators had a lower lymphocyte response to phytomitogens compared to rapid acetylators.