Somatic cell hybrid clones were derived from the fusion of hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8)-deficient mouse cells and two different human fibroblast strains, each carrying an X chromosome-autosome translocation. One of these had an X/11 translocation [46,X,t(X;11)(p21;q13)] and the other had an X/19 translocation [46,X,t(X;19)(q22;q13)]. The structurally normal human X chromosome is the late-replicating (genetically inactive) chromosome in these two cell strains; the rearranged X chromosome is early replicating (genetically active). One primary hybrid clone carrying both the translocated X chromosome and the structurally normal X chromosome was isolated in hypoxanthine/aminopterin/thymidine medium from each of these two cell fusion experiments. These clones were then selected in medium containing 8-azaguanine to achieve the loss of the active human HPRT locus. Five subclones from the cell hybrid with the X/11 translocation failed to express two known human X-chromosome markers [glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49) and phosphoglycerate kinase (PGK; EC 2.7.2.3)] but did express human microsomal steroid sulfatase (STS; sterol-sulfate sulfohydrolase, EC 3.1.6.2). Three of these were cytogenetically analyzed and found to contain a structurally normal human X chromosome but not the X/11 translocation. Two subclones were isolated in 8-azaguanine from the hybrid with the X/19 translocation. Cytogenetic analysis of these two clones showed the presence of a structurally normal human X chromosome; the X/19 translocation was not present. They did not express human G6PD, PGK, or HPRT but did express human STS. These results indicate that human STS is expressed from a locus on the inactive human X chromosome and support our earlier finding that the STS locus escapes X-inactivation in man.