An antibody of hitherto unknown specificity has been found in the serum of a patient who suffered three episodes of agranulocytosis after ingestion of aminopyrine derivatives. In vitro, the antibody reacts not only with homologous and autologous granulocytes and thrombocytes. It belongs to the IgM class and absorption of the serum with earlier leukocytes or platelets eliminates the activity against all three cell types. An HL-A specificity (LA or Four locus) can be ruled out, since all tests are positive with cells of individuals differing from the patient in all four HL-A specificities. Addition of the patient's serum results in marked reduction of thymidine uptake in the mixed leukocyte culture as well as during stimulation of lymphocytes with phytohemagglutinin or PPD; however, almost no effect is observed on stimulation with small amounts of Pokeweed mitogen. The rosette formation of lymphocytes with sheep erythrocytes or sensitized human erythrocytes is not influenced by this serum. This pattern is comparable to what is known for alpha-methyl-dopa-induced autoimmune hemolytic anemia insofar as the antibody does not require the presence of the offending drug for its action.