Human placental mono-oxygenase activities were markedly increased after additions of micromolar quantities of hematin to reaction vessels. The magnitude of the increases diminished with increasing (induced) levels of hematin-independent activity. The activating effect of hematin could be observed in unbroken cell preparations, in whole homogenates, and in various subcellular fractions. Highest hematin-dependent activity was measured in microsomal fractions of placental homogenates. With benzo(a)pyrene as substrate, response to the stimulatory effect of hematin in human placental preparations was not as profound as that observed in monkey or rabbit placentas but was more marked than the responses observed in placental preparations from rats or mice. Hematin-activated mono-oxygenase activity present in washed microsomal fractions of human placental homogenates could be solubilized with detergents, the most effective of which was Triton N-101. The solubilized activity also could be partially purified by polyethylene glycol fractionation. Attempts to further purify, however, resulted in loss of activity. All results were consistent with the hypothesis that the effect of hematin is mediated via reconstitution of hematin-free apocytochrome(s) P-450.