Cells from regenerating mouse liver removed 2-25 days post 68% hepatic resection have been assayed for in vivo colony forming capacity in soft agar (CFU-C), proliferative capacity in liquid culture, and in vivo spleen colony forming capacity (CFU-S). These studies demonstrated low concentrations of CFU-C and CFU-S in normal and sham operated liver, with an appreciable increase of both in regenerating liver, reaching maximum values in tissue removed 5--7 days post hepatic resection. Colony formation in agar by regenerating liver cells occurred in the absence of exogenous colony stimulating factor. Separation of liver cells on the basis of adherent properties prior to culture indicated concentration of CFU-C in the non-adherent fraction, while cells producing colony stimulating factor were concentrated in the adherent fraction. Foci of actively dividing cells of themacrophage and granulocyte series arose in liquid culture from preparations of sham operated and regenerating liver, although total cell formation was greater with regenerating liver. A small proportion of the colonies formed in agar from regenerating liver consisted of cords of epithelioid cells, which resembled hepatocytes and differed from the macrophages or granulocytes found in the majority of colonies, raising the possibility that regenerating hepatocytes form colonies in agar culture.