The cellular origin of tumors induced by the chemical carcinogen 3-methylcholanthrene (MCA) was studied in mice with X-chromosome inactivation mosaicism. Because only one of the two X-chromosomes is active in XX somatic cells, a female heterozygous at the X-linked phosphoglycerate kinase (PGK-1) locus for the usual Pgk-1b gene and the variant Pgk-1a has two populations of cells, in the cells of one population, Pgk-1b is active and B-type enzyme is synthesized, whereas in cells of the other population, A-type enzyme is produced. Both enzyme types are found in normal tissues from these mosaic mice. A tumor developing from a single cell exhibits only one of the two PGK enzyme types, whereas a tumor with a multicellular origin expresses both enzymes (i.e., it has a double-enzyme phenotype). Five fibrosarcomas developing at the site of injection of 0.2 or 2.0 mg of MCA were analyzed. 36 of 38 fragments from the five tumors had double-enzyme PGK phenotypes. One piece from each of two tumors showed a single-enzyme phenotype. Histological, cell culture, and cloning studies indicate that the double-enzyme phenotypes reflect the presence of both types of malignant cells and not admixture of normal with neoplastic elements in the specimens tested for PGK. The results suggest strongly that these fibrosarcomas have a multicellular origin.