After i.v. injection of glucans (yeast cell walls, lentinan and pseudonigeran) into CBA mice, the peritoneal macrophages became cytostatic to R1 leukemia cells in vitro, and the mice showed resistance to lung nodule formation by i.v. injected T3 fibrosarcoma cells. The glucans were less active than C. parvum. None of the glucans reproducibly acted as an adjuvant when mixed with irradiated tumour cells; nor did they inhibit the growth of a subcutaneous tumour when injected i.v. or directly into the tumour.