Aryl hydroxylase activity has been demonstrated to depend on the pattern of tumor cell structural organization. The activity of microsomal monoxygenases in the ascitic forms of sarcoma MC-11, hepatoma 22a and Ehrlich's tumor was much lower than in the corresponding solid tumors. Aryl hydroxylase was activated after the animals received 3-MC, but the magnitude of the activity induced did not correlate with the basic activity in the different tumors. In in vitro experiments, 7,8-benzoflavone inhibited the enzyme in all the tumors, whereas metyrapone did not affect BP-hydroxylation. It is assumed that all the tumors investigated contain hemoprotein that is similar to cytochrome P1-450.