AbstractThe number of cells from which tumors induced by neonatal injection of Rauscher Leukemia virus (RLV) develop was investigated using F1 (C57BL/6 X Feral) female mice heterozygous at the X‐linked phosphoglycerate kinase (PGK) locus. Because of inactivation of one of the two X chromosomes in each somatic cell which occurs during embryogenesis in female mice, only one of the two PGK genes is active in each somatic cell. Thus, tumors that develop clonally in these mice exhibit one enzyme type, B or A, whereas those with a multicellular origin may exhibit both enzymes. Seventeen of 22 PGK heterozygous mice injected with RLV developed lymphosarcomas after an average latency of 38 weeks. Twelve of these lymphomas exhibit a single‐enzyme type, A or B. In three of five tumors with double‐enzyme phenotypes, the minor enzyme component comprised 20% or less of the total PGK activity, and histological examination revealed that these tumors contained a mixture of malignant and normal‐appearing cells, suggesting that the minor PGK component was not contributed by the malignant cells. In the two remaining lymphomas, the ratio of A:B PGK activity was close to l:l. However, lymphomas which developed in recipients after transplantation of cells from these two tumors into newborn mice showed only single‐enzyme PGK phenotypes, suggesting that the neoplastic cells in the primary tumors had single‐enzyme phenotypes. Thus, these results indicate that lymphosarcomas induced by RLV in mice develop clonally.