The sera from 38 patients with suspected drug-induced thrombocytopenia (22) or neutropenia (16) were tested with the indirect immunofluorescence test on platelets or granulocytes for the presence of drug-dependent antibodies. Three drug-induced antibodies with reactivity against platelets and 5 with reactivity against granulocytes were detected. In 3 sera antibodies were found which reacted already with target cells without adding the drug to the test system. These data show that drug-induced blood dyscrasias often have an immunological cause and that in vitro tests can be helpful in detecting the responsible drug. Different mechanisms can be involved. In many sera circulating antibodies were not found, but an immunological mechanism is likely to be involved in some of these cytopenias: the antibody could be entirely absorbed by the target cells, a metabolite of the drug could be the immunogen and finally the test may not be sensitive enough.