Effects of intraperitoneal (i.p.) administration of SSG, a (1----3)-beta-D-glucan obtained from the culture filtrate of Sclerotinia sclerotiorum IFO 9395, on hematopoietic responses of mice were investigated. Numbers of spleen and peripheral leukocytes increased 7 d after administration of SSG (250 micrograms/mouse). The percentages of polymorphonuclear leukocytes (PMN) in both spleen and peripheral blood increased markedly. The numbers of macrophage progenitor cells also increased in both spleens and femurs of the mice administered SSG. Furthermore, levels of colony-stimulating activity (CSA) in sera were elevated, and two peaks were observed (at 6 h and on day 7 after administration). Elevated CSAs were also observed in the cultures of peritoneal cells taken from mice 3 or 6 h after administration of SSG. These results may suggest that i.p. administration of SSG into mice enhanced the production of colony-stimulating factors (CSFs), and then increased the numbers of both spleen and peripheral blood leukocytes.