Mononuclear phagocytes (monocytes and macrophages) are widely distributed throughout the body. Indium-111 (111In) oxine has been a useful radioactive label in studies of in vivo cellular kinetics and distribution. In preparation for in vivo monocyte investigations in humans, this study was designed to determine the effects of labeling human blood monocytes with 111In on their function in vitro. The monocytes labeled with an efficiency of 57.9 +/- 14.2% (range of 47-95%). The 111In eluted from the cells at a rate of 0.25%/h over 48 h in vitro. The labeled monocytes had a slight, but statistically significant, reduction in their ability to phagocytize or mediate antibody-dependent cellular cytotoxicity of antibody-coated sheep erythrocytes. The labeled cells were not different than control monocytes with respect to viability (trypan blue exclusion and lactate dehydrogenase release), adherence to plastic, hydrogen peroxide production, chemotaxis to N-formyl methionylleucylphenylalanine, and antibody-independent lysis of the tumor cells U937 and HeLa. 111In-labeled human monocytes should be useful for in vivo studies of mononuclear phagocyte kinetics and distribution in normal and diseased subjects.