Gold agents have been widely used for the treatment of rheumatoid arthritis. We studied the growth inhibiting effect of such an agent on malignant cells in vitro. HCT-15, AGS cells derived from a human malignancy, and Meth/A cells from a malignant lymphoma of Balb/C mice were cultured separately with gold agent at concentrations of 2 micrograms/ml. Four days after the cultures had been incubated in a 5% CO2 incubator at 37 degrees C, cell counts were made; and significance of differences was analyzed by Student's t test. Additionally, HCT-15 cells were cultured with gold for two days, and then the cells were analyzed by flow cytometry. The growth of HCT-15, AGS, and Meth/A cells was suppressed by gold. Fifty percent suppression was observed at a concentration between 50 micrograms/ml and 10 micrograms/ml for HCT-15 cells, between 125 micrograms/ml and 50 micrograms/ml for AGS cells, and between 125 micrograms/ml and 50 micrograms/ml for Meth/A cells. Fifty percent suppression of HCT-15 cell growth by cisplatinum was found between 50 micrograms/ml and 10 micrograms/ml. Flow cytometric findings showed a significant rise in the tetraploid peak, a mild rise in the resion between diploid and tetraploid peaks, and an increase in cells with a ploidy greater than four. These data suggest that gold blocks the S phase, G2 to M phase, and M phase as well. To observe the cytotoxicity of gold, each of 10 of 4 week-old Balb/C mice was injected s.c. at a dose of 10 mg/kg or 2 mg/kg every other day for a total of 3 injections, or was administered the gold at 30 mg/kg/day p.o. for 10 days. All mice were still alive after 20 days of observation. Cisplatinum at a dose of 10 mg/kg was also injected s.c. one time into each of 10 mice, and 60% of the animals died within 10 days after the injection.