The cytotoxic activity of polymorphonuclear leukocytes (PMN) against tumor cells induced in vitro by antitumor immunomodulators was examined by a 51Cr release cytotoxicity assay. Among 28 immunomodulators and other agents thus far tested, only beta-1,3-glucan from Alcaligenes faecalis var. myxogenes IFO 13140, Bacillus Calmette-Guérin, Propionibacterium acnes, zymosan A, and Nocardia cell wall skeleton were found to cause induction. The cytotoxic activity of PMN with the beta-1,3-glucan was very high, almost 100% cytolysis being observed at an effector:target ratio as low as 3. The other four potent immunomodulators had effects very similar to that of the beta-1,3-glucan. All five tumor cell lines tested, MM46, MM48, MH134, EL-4, and YAC-1, were lysed, whereas normal spleen and thymus cells and PMN were not. Of four types of effector cells tested, PMN and casein-induced macrophages were effective, whereas resident macrophages and J774 cells were not effective. The cytotoxic activity of PMN was greater than that of induced macrophages, although both were induced by casein. From results on the polysaccharides tested, a linear beta-1,3-glucan structure and a minimum number average degree of polymerization of 125 of the beta-1,3-glucan seemed to be required for induction of cytotoxicity of PMN. The cytotoxic features of PMN and possible chemical structures of antitumor polysaccharides for induction of cytotoxicity are discussed.