The morphological and biochemical consequences of treatment of human peripheral blood leukocytes with cytochalasin B were studied. Incubation of human polymorphs with cytochalasin B resulted in nuclear and cytoplasmic spreading, but not in spontaneous release of lysosomal enzymes. Cytochalasin B inhibited particle uptake. Consequently, phagocytic vacuoles were not observed; instead, granule contents were discharged directly into the surrounding medium when cytochalasin B-treated cells were challenged with zymosan particles. Cytochalasin B enhanced the release of lysosomal enzymes from human polymorphonuclear leukocytes whether these encountered zymosan particles or immune complexes on a nonphagocytosable Millipore filter. Cytochalasin B-treated leukocytes thus constitute a model system for quantitative study of lysosome fusion. Augmented enzyme release was blocked by prior treatment of cells with pharmacological doses of agents that influence the accumulation of cyclic nucleotides (cyclic nucleotides themselves, prostaglandin E(1)) or by compounds that interfere with microtubule function (e.g., colchicine, vinblastine). These observations suggest that one action of cytochalasin B on phagocytic cells is to remove the normal constraints to merger of granules, either with each other or with the plasma membrane, and that intact microtubule function is required for translocation of lysosomes.