Among 37 patients treated with levamisole for rheumatoid arthritis (n = 19), for Reiter's disease (n = 4) and for chronic articular brucellosis (n = 14) followed up during 6-12 months, 3 developed agranulocytosis and 3 severe neutropenia. Serum samples drawn before and during treatment were tested for leukocyte agglutinating and lymphocytotoxic antibodies. Leukocyte agglutinating antibodies were induced in 8 patients, in 5 of them in association with agranulocytosis or neutropenia. In 1 patient with agranulocytosis and in another one with neutropenia lymphocytotoxic antibodies were also induced. Two agranulocytotic and one neutropenic patient possessed HLA B27 antigen. In altogether 11 HLA B27 carriers the number of circulating neutrophils were significantly reduced during levamisole treatment when compared with those of patients lacking HLA B27 antigen.