Treatment of rat hepatoma dRLh-84 cells with sodium arsenite, cadmium chloride and cobalt chloride resulted in marked induction of protein with a molecular mass of 32 kDa. To examine the possibility that the induced 32 kDa protein may be heme oxygenase, the enzyme activity was measured, and then the activity in the cells increased by these metals, and heat shock treatments. Immunoblot analysis showed that the induction of 32 kDa protein reacted with anti-heme oxygenase antibody occurred by the treatments. Northern blot analysis confirmed that the induction of heme oxygenase by arsenite, cadmium and cobalt, and by hyperthermia was regulated at transcriptional level. These findings support the hypothesis that the 32 kDa stress protein induced by heavy metals in rat hepatoma cells is heme oxygenase.