The expression of steroid sulfatase (SS; sterol-sulfatase; sterol-sulfate sulfohydrolase, EC 3.1.6.2), a microsomal enzyme that catalyzes the hydrolysis of a variety of 3beta-hydroxysteroid sulfates, was evaluated in mouse-human hybrid clones. The mouse parental line, A9, was found to have little SS as determined by activity measurements. Human SS can be separated from mouse SS by electrophoresis. Two human fibroblast lines, one carrying an X/13 translocation [46,X,t(X;13)(p22;q12)] and the other carrying an X/20 translocation [46,X,t(X;20)(Xp20q;Xq20p)] were used as the human parental lines. Several independently derived hybrid clones from each of the two fusion experiments were analyzed for the expression of human SS by activity measurements and electrophoresis. Cytogenetic analyses were done on these hybrid clones at the same passage level. The results showed that the expression of human SS in these cell hybrids was concordant only with the presence of the distal half (p22-->pter) of the short arm of the human X chromosome, thus assigning the locus for SS to Xp22-->Xpter. Earlier studies have shown that the deficiency of SS is the basis for the dermatologic condition X-linked ichthyosis, the gene for which is known to be located approximately 10 centimorgans from the Xg blood group locus. The localization of SS on the X chromosome indicates that Xg locus may be on the short arm of X and possibly on its distal half. The Xg locus is thought to escape X-inactivation in man, and recent investigations suggest that the SS locus also escapes X-inactivation. Our results thus provide evidence for the location of an apparently noninactivated site on the distal half of the short arm of the human X-chromosome that contains the locus for SS and possibly the Xg locus.