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The effects of various modifiers upon the interaction of LPS- and BCG-activated macrophages with cells of mastocytoma P815 have been investigated. The efficiency of binding and lysis of the tumor cells is to a great extent determined by activation of the effector-cells, expression of the trypsin-sensitive receptors on the surface of macrophages, and by the type of target-cells. Introduction into the analytical system (effector-target) of unlabeled tumor cells or membrane preparations obtained from them inhibits substantially both binding and lytic activity of cytotoxic macrophages. If nontransformed cells or their membranes are applied, no significant changes in the investigated processes can be detected. Trypsinization of tumor cells as well as of activated but not resident macrophages modifies considerably the interaction of effectors with targets. The quantity of tumor cells bound with macrophages does not depend on the fact, which of the partners is subject to trypsinization, but it is much less than that of target-cells bound in the control. The incubation of activated macrophages with actinomycin D results in a substantial suppression of their lytic activity, whereas treatment of tumor cells with this inhibitor of protein synthesis leads to a considerable decrease in stability of the targets against lytic activity of the factor activated by effectors. The obtained data reveal the ways of selective binding and effective lysis of transformed targets by activated macrophages.