The functions of peripheral blood monocytes in 14 untreated patients with lung cancer were examined before and after a 1-week course of OK-432 therapy(1 KE every day). The absolute monocyte count was calculated from the percent of monocytes in the total leukocyte count. The mean absolute numbers of monocytes before and after administration of OK-432 were 400/cmm and 437/cmm, respectively though the difference was not statistically significant. The lysosomal enzyme activity of monocytes was histochemically determined by betagalactosidase staining. The mean percentages of beta-galactoside positive monocytes before and after administration of OK-432 were 31.2 and 47.7 respectively. The increase was significant(P<0.05). The monocyte chemotactic response was measured in vitro by Boyden's millipore technique using zymosan activated human serum as the chemoattractant. The monocyte chemotactic responses before and after administration of OK-432 were 20.1 and 25.2, respectively and a significant difference was found between the two groups(P<0.01). The ability of monocytes to phagocytose complemented zymosan was accelerated by OK-432 administration, but the acceleration was not statistically significant. Cytostatic activities of monocytes on EBC-1 target cells before and after administration of OK-432 were 84.5 and 91.4, respectively the difference was significant(P<0.05). These results indicate that OK-432 activates the reduced monocyte functions in lung cancer patients in vivo, and that OK-432 may be a useful immunotherapeutic agent in man.