Conjugation of a number of steroids with sulfuric acid has been determined quantitatively in mammary cancer, normal mammary tissue, liver metastases and normal liver tissue. Steroid sulfokinase activity has been demonstrated in primary mammary cancer in 10 patients but is not measurable in 4. The finding suggests that there are 2 populations of mammary cancer cells, one which possesses no enzyme activity and one which contains enzymes capable of forming sulfate esters with a number of steroid alcohols. The normal mammary tissues appear to contain only weak steroid sulfate-forming activity if any at all. Normal liver tissue is highly active in conjugating steroid hormones. Although various liver preparations vary considerably in their total ability to sulfurylate different steroids, the relative activity is very close to being constant; DHEA was the most efficiently sulfurylated, and the ratio of each of the other steroids to DHEA is essentially the same. The pattern of sulfurylation of steroid hormones by mammary cancer tissue is not the same as that of the liver. In mammary cancer tissue, conjugation of estradiol does not follow a constant pattern of efficiency of sulfurylation as by the liver tissues. Some mammary cancer sulfurylate DHEA more efficiently than estradiol-17β and others do the reverse. The ability to form steroid sulfate by the neoplastic liver tissues does not seem to follow the pattern of either the normal liver counterpart or the primary neoplastic tissue. This novel observation suggests that primary and metastatic mammary cancers may differ in their ability to conjugate steroid hormones.