A line of AKR leukemic mouse cells cultured in vitro, designated AKR 87, was established and characterized. Two types of cells were distinguished in this line fibroblasts and lymphoblasts. The lymphoblast-like cells resembled the original tumor cells spontaneous leukemia of AKR mice. In the XC test, cells of this line reacted specifically with XC cells, forming syncytia characteristic of cells infected with mouse leukemia viruses. Ultrathin sections of cells of this line examined on the electron microscope showed presence of type C viral particles.