Heme oxygenase is rate-limiting in the heme degradative pathway, and its activity is induced by a host of chemicals. In K562 human erythroleukemic cells, heme oxygenase activity was not increased by exposure to potent inducers, such as cobalt chloride, bromobenzene, and heme. Indeed heme treatment severely suppressed the enzyme activity, and at 18 h the activity measured less than 5% of the control. Heme and cobalt chloride did not inhibit activities of NADPH-cytochrome c (P-450) reductase and biliverdin reductase to a marked degree. In contrast, treatment of cells with thymidine/hypoxanthine alone, or in combination with cobalt chloride, caused an increase in the activity of three enzymes of heme degradation. It is suggested that with thymidine, which is a committing inducer of hemoglobin synthesis, the induction of activity of the three enzymes of the heme degradation pathway is coupled with cell differentiation. On the other hand, in the case of heme, a noncommitting inducer of hemoglobin synthesis, induction of hemoglobin synthesis and increase in heme degradation activity may be independent.