Primary cultures of adult rat hepatocytes have been utilized to investigate the role of calcium as a mediator of carbon tetrachloride (CCl 4) and bromotrichloromethane (CBrCl 3) hepatotoxicity. Two distinct toxic responses can be observed using this system: (1) lifting of attached cells from the monolayer without release of cytoplasmic enzymes into the culture medium and (2) leakage of free cytoplasmic enzyme into the medium. CCl 4 and CBrCl 3 are metabolically activated in these cells resulting in extensive toxicity expressed as cell lifting from the monolayer, enzyme leakage, and gross changes in cell morphology including blebing and ballooning of the cell membrane. In calcium-free culture medium the extent of enzyme leakage and membrane ballooning is reduced without any apparent change in the metabolic activation of halomethane. In the presence of physiological concentrations of calcium in the culture medium, the ionophore A23187 causes enzyme leakage and blebing of the hepatocyte surface, similar to the morphological changes seen with halomethane, possibly by allowing calcium to accumulate in the cell. In addition, the ionophore potentiates CCl 4 toxicity but only in the presence of physiological calcium concentrations. These results are consistent with the view that CCl 4 may increase the concentration of free calcium in the cytoplasm of the cell, resulting in the observed cell damage.