Recombinant human-interferon-gamma (rH-IFN-gamma) and verapamil (VRP), either alone or in combination, were evaluated in MTT assay for their modification effects on adriamycin-induced cytotoxicity against MKN-45, human stomach adenocarcinoma cells. VRP as a single agent did not inhibit the survival of MKN-45 at doses of up to 5.0 micrograms/ml. The survival of MKN-45 was inhibited by rH-IFN-gamma dose-dependently and further inhibited by the addition of VRP. However, the maximum growth inhibition of MKN-45 in any combination treatment with rH-IFN-gamma and VRP was less than 50% except in the highest concentration combinations (% survival: 47.9% at 10(4) U/ml of rH-IFN-gamma and 3.0 micrograms/ml of VRP). Adriamycin caused a concentration-dependent cytotoxicity and its cytotoxicity was significantly enhanced by the addition of rH-IFN-gamma and further enhanced by the combined use of rH-IFN-gamma and VRP. The modification effects of rH-IFN-gamma and VRP on adriamycin-induced cytotoxicity were evaluated in terms of modification index (MI), demonstrating that rH-IFN-gamma significantly increased in adriamycin-induced cytotoxicity and that the combined use of rH-IFN-gamma and VRP enhanced the adriamycin-induced cytotoxicity to a greater extent than did rH-IFN-gamma alone: MI values at 10(2) U/ml and 10(3) U/ml of rH-IFN-gamma were 1.7 and 3.1, respectively; those at 1.5 micrograms/ml and 3.0 micrograms/ml of VRP in the presence of 10(3) U/ml of rH-IFN-gamma were 4.4 and 6.0, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)