Normal human polymorphonuclear leukocytes (PMNL) were preincubated in vitro with methotrexate, 5-fluorouracil, vincristine, cisplatin, Adriamycin (doxorubicin), and daunomycin for 15 hours before being tested in a phagocytic-bactericidal assay. Anthracycline-treated PMNL were defective in phagocytosis and killing of the bacteria, in contrast to the other chemotherapeutic agents which allowed the PMNL to remain functional. The defect of Adriamycin-treated PMNL resulted from decreased ingestion: 3 micrograms/ml Adriamycin inhibited by 50% of the uptake of Oil Red-O particles. In this assay, the proportion of noningesting PMNL increased from less than or equal to 20% with 0.62 micrograms/ml to greater than or equal to 90% with 10 micrograms/ml Adriamycin. Electron microscopy revealed that Adriamycin-inactivated PMNL had rounded up, were depleted in glycogen, and had undergone profound nuclear changes. RNA and protein synthesis in PMNL were also affected. Adriamycin, besides producing neutropenia, may decrease the phagocytic function of circulating PMNL.