Highly purified (97%-99%) and viable (99%) peripheral blood monocytes obtained by EDTA-reversible adherence to autologous-serum-precoated plastic surfaces could rapidly lyse a variety of tumor cells in a 3–4 hr 51Cr release assay. Using these monocytes as effectors, a short-term agarose/conjugate assay was utilized, permitting us to examine the interaction between fresh human monocytes and neoplastic target cells on a single cell level. That the tumor-bound effector cells were indeed monocytes was confirmed by employing the monocyte-specific monoclonal antibody 61D3, which stained 95%-99% of the mononuclear cells bound to conjugated and killed K562 tumor targets. The binding of monocytes to target cells appeared to be temperature dependent and was extremely rapid, reaching a plateau after 5 min at 30 degrees C. Our findings demonstrated for the first time that only a proportion of human blood monocytes can bind to a particular target cell and that only a fraction of the binding cells have the intrinsic potential to kill those neoplastic targets. The proportion of monocytes capable of binding and killing varies between individuals and also depends on the tumor cell used, indicating heterogeneity in the monocyte and tumor cell populations. The highest proportion of monocytes bind to the human erythromyeloid leukemia K562 cell line (13%-50%). The frequency of monocytes capable of killing K562 tumor cells is relatively low (7%- 13%). The system described here should be useful to study the heterogeneity of mononuclear phagocytes and to analyze the molecular basis of the interaction between those effector cells and neoplastic target cells.