A series of experiments was conducted to examine the effect of glucan on the reticuloendothelial system (RES) and on the development of 90Sr-induced osteosarcomas and malignant lymphomas in CBA/S mice. Glucan demonstrated a strong RES-stimulating effect, as evidenced by a dose-related increase in lysozyme levels in the plasma and an enlargement of the liver and spleen. Weekly injections of glucan between 150 and 250 days after exposure to 90Sr suppressed the actuarial appearance of the fibroblastic type of osteosarcomas and stimulated the emergence of malignant lymphomas. Glucan itself had no tumorigenic effect in mice not exposed to 90Sr.