DNA containing the herpes simplex virus thymidine kinase (HSVtk) gene was used to transform wild-type tk+ mouse L cells to a tk++ status in vitro using methotrexate as a selective agent. HSVtk DNA was also used to transform mouse bone marrow cells in vitro. Transformed marrow cells injected into irradiated and methotrexate-treated recipient mice gave rise to proliferating cells which in some cases dominated the marrow population and which contained HSVtk gene sequences.