Cell density (no. of cells per unit area or volume) during drug treatment may play a role of considerable importance in the interpretation of drug toxicity experiments performed in vitro. Chinese hamster V-79 and mouse L-929 cells exposed to nitroheterocycles under aerobic conditions are considerably more sensitive to the cytotoxic effects of these drugs when incubated at low cell density (10(2) cells/cm2 or 10(4) cells/ml) than at higher cell density (10(4) cells/cm2 or 10(6) cells/ml). This may be related to diffusion limitations when cells are in contact and to the ability of dense cell suspensions to inactivate drugs. In contrast, under anaerobic conditions, more toxicity is observed at high cell density than at low cell density, perhaps due to local effects of toxic metabolites. Toxicity appears to correlate with intracellular drug levels under both aerobic and hypoxic conditions.