The influence of eight antimyeloid monoclonal antibodies on human leukocyte phagocytosis was investigated using flow cytometry. A granulocyte-specific monoclonal antibody, VIM-D5, inhibited the phagocytosis of both zymosan particles and Staphylococcus aureus in a dose-dependent fashion. In the presence of 5 micrograms/ml, the numbers of phagocyte-associated zymosan particles and bacteria were reduced by about 35% and 40%, respectively. Another monoclonal antibody, VIM-12, reacting with granulocytes, monocytes, and null lymphocytes, inhibited both granulocyte and monocyte phagocytosis of S. aureus. The inhibition was dose dependent, and in the presence of 10 micrograms/ml, the number of phagocyte-associated bacteria was reduced by about 40%. VIM-12 did not influence the phagocytosis of zymosan particles. Both VIM-D5 and VIM-12 inhibited the internalization phase of phagocytosis, whereas the attachment to the phagocyte surface was unaltered. The combined effect of VIM-D5 and VIM-12 was additive, amounting to about 70% reduction of phagocytosis of bacteria. The remaining six antimyeloid antibodies had no effect on leukocyte phagocytosis. The combined use of antimyeloid monoclonal antibodies and flow cytometry appears to be a promising tool for the study of phagocyte functions.