SUMMARY A microgranulocyte cytotoxicity test was developed and tested. Granulocyte adhesiveness and consequent difficulty in microscopically distinguishing live and dead cells by conventional stains was the most difficult problem. Pretreatment of granulocytes with iodoacetamide permitted use of standard microcytotoxicity methodology. Granulocyte cytotoxins in the absence of lymphocyte cytotoxins were found in 110 of 602 sera tested for both kinds of antibodies. Of 91 sera from patients with rejected kidney transplants, 89% contained granulocyte cytotoxins. The presence of granulocyte cytotoxins in the serum of patients before transplantation was also significantly associated (P < 0.05) with subsequent failure among the 214 transplanted patients studied. Some data on the time of appearance of granulocyte cytotoxins in relation to lymphocyte cytotoxins are also given.