A potent inhibitor of protein kinase C(PKC), 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), dose-dependently inhibited natural killer (NK) activity in large granular lymphocytes (LGL) pretreated at 37 degrees C for 30 min. However, neither N-(2-aminoethyl)-5-isoquinolinesulfonamide dihydrochloride (H-9), which inhibits more effectively cyclic nucleotide-dependent protein kinases than other kinases, nor N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (HA1004), which was used as a control for H-7, reduced NK activity. The inhibitor effect of H-7 was not due to changes in effector cell viability or target cell binding. We also found that H-7 suppresses PKC activity in both the cytosol and membrane fractions of LGL. From these findings, PKC is considered to play an essential role in the lytic mechanism of NK cell-mediated cytolysis.