Intact sheep red cells potentiated mouse B lymphocyte colony growth in agar but red cell membranes or lysates exhibited no comparable ability. For maximum colony formation red cells had to be present throughout the entire culture period. Red cells added to cultures late in the culture period did not potentiate growth. Intimate contact between colony-forming cells and red cells was not essential for potentiation to occur. Red cell lysates inhibited normal B lymphocyte colony formation in cultures containing intact red cells, but did not inhibit colony formation by cells of the B lymphoid leukemia ABE-8. This differential effect may provide a means of differentiating normal from neoplastic colony-forming B lymphocytes in the mouse. Eight different tumors were also examined. Intact red cells potentiated colony formation by all of them. Lysed red cells did not potentiate the growth of any of the tumor lines. The mastocytoma P815 was the only tumor whose colony formation was inhibited by the addition of intact and lysed red cells.