The acute toxicity of tetravalent platinum was studied in vitro by use of rabbit alveolar macrophages and human lung fibroblasts (strain WI-38). Alveolar macrophages were exposed in tissue culture for 20 hr to platinum dioxide (PtO2) or platinum tetrachloride (PtCl4). There was no evidence of dissolution of PtO2 and no decrease in viable cells at concentrations as high as 500 mug/ml. PtCl4 was soluble in the macrophage system and after a 20-hr exposure, resulted in loss of viability in 50% of the cells originally present at a concentration of 0.30mM (59 mug Pt/ml). After a 20-hr exposure, rapidly growing human lung fibroblasts were rendered nonviable by PtCl4 at comparable concentrations. A decrease in total cellular ATP was observed at lower concentrations in macrophages and fibroblasts along with a reduction in phagocytic activity of macrophages as compared to controls. With the fibroblasts, a 50% decrease in incorporation of 14C-thymidine was observed after a 22-hr exposure to PtCl4 at a concentration of 0.007mM; higher concentrations were required to inhibit the incorporation of 14C-uridine and 14C-leucine. Time-course studies indicated that the inhibition of 14C-thymidine incorporation was nearly complete (90%) after 7 hr in the presence of 0.06mM PtCl4. Under the same conditions, there was little inhibition (15%) of 14C-leucine incorporation and moderate inhibition (50%) of 14C-uridine incorporation. Higher concentrations of PtCl4 were required to inhibit 14C-thymidine incorporation into the acid-soluble fraction than were required to inhibit incorporation into the acid-precipitable fraction. Hence, the preferential inhibition of DNA synthesis by PtCl4 may result from an impairment of the incorporation process.