The cytotoxic concentrations of about 100 randomly selected drugs and chemicals, tested on HeLa cells in the MIT-24 system and/or in primary cultures of fetal chicken cells, were compared with the lethal doses and/or concentrations of the agents in the mouse and in man. Most agents (80%) had a similar toxicity in vitro and in vivo, suggesting a lethal interference in man with basal functions common to all specialized human tissues as well as cultured cells, i.e., basal cytotoxicity. This high frequency of basally cytotoxic agents opens possibilities for screening chemicals for toxicity and for studying cytotoxic mechanisms with a standard battery of a few appropriate cell tests. This battery may be used in three ways: (1) to study cytotoxic mechanisms of all chemicals, and apply the resulting knowledge to understanding toxicity in man of basally cytotoxic agents; (2) to supplement conventional animal tests in acute toxicity test programs; (3) to screen chemicals and extracts for their potential basal cytotoxicity. To validate these ideas and to select suitable tests for the battery, results from cytotoxicity tests on a wide variety of chemicals in several in vitro systems must be compared with one another and with the toxicity of the agents in animals and man.