A new murine myeloid leukemia cell line (C2M) with A-type phosphoglycerate kinase (PGK) as marker isoenzyme was established from myeloid leukemia which arose in a female C3H/He strain mouse of the genotype Pgk-1a/Pgk-1b 1 yr after a whole body X-irradiation of 3 Gy. Cytochemical stainings indicated that C2M cells had myelomonocytic characteristics. Chromosomal analysis showed the partial deletion of No. 2 chromosome. Intravenous injection of C2M cells resulted in the development of myeloid leukemia in syngeneic mice owing to the growth of C2M cells. When C2M cells were transplanted to C3H/He mice with B-type PGK, PGK of spleen expressed two bands on electrophoresis; A-type PGK from transplanted C2M cells and B-type PGK from recipient mice, and the density of A-type PGK became prominent as the disease progressed. When granulocyte/macrophage progenitor cells of bone marrow cells from leukemic mice were cultured, two types of colonies were observed. By determining PGK types of the colonies, leukemic colonies could be differentiated from normal granulocyte/macrophage colonies. Since C2M cell line has an advantage of processing A-type PGK which can be readily distinguished by the electrophoresis from normal cells, it will serve as a useful tool to study the interaction between leukemic cells and normal hematopoietic cells.