The ability of hydrocortisone to induce two enzymes in 9 cancer cell lines and 6 normal cell lines was investigated. Amino acid naphthylamidase was induced in 4 out of 5 malignant cell lines of high activity, and not in 4 lines with very low activity. No induction was achieved in the normal cell lines. The results on alkaline phosphatase were variable but excluded the possibility that the normal and malignant cell lines differed with respect to a steroid receptor.