In vitro colony-forming cells (CFUc) were evaluated in preleukemic and leukemic AK mice. Increased concentrations of CFUc were found in normal appearing marrow of superinfected animals 2–3 wk prior to the onset of lymphoma. CFUc were present in marrows of mice with virus-accelerated, spontaneous, and transplanted lymphoma. CFUc concentration was often increased in mice with advanced disease and marrow replacement by lymphoblasts. When calculated on the basis of CFUc per normal marrow cells, this increase was marked. The colonies developed only in the presence of added colony-stimulating activity (CSA) and had the morphologic features of colonies from normal marrow. Leukemic cells did not form colonies. Lymphoma cells, from virus accelerated, spontaneous and transplanted lymphoma, did not produce CSA in feeder layers or conditioned medium. Leukemic and nonleukemic AK bone were found to produce similar small amounts of CSA. These studies showed that the preleukemic state as well as marrow replacement by lymphoblasts resulted in increased marrow CFUc's. No evidence for increased local production of CSA by lymphoblasts or the marrow microenvironment was found to account for this.