The effect of lentinan on granulopoiesis in BALB/c nude mice (Nu/Nu) was compared with that in their heterozygotes (Nu/+). Lentinan-injected Nu/+ showed enhanced serum colony-stimulating activity (CSA), and increased diphasically granulocyte-macrophage colony forming cells (GM-CFC) in bone marrow and spleen (i.e., first peak on day 1-2 and second peak on day 6). On the other hand, in Nu/Nu, lentinan did not enhance serum CSA, although the control level was higher than the enhanced level in Nu/+. Monophasic increase in GM-CFC was observed in Nu/Nu, but the second peak seen in Nu/+ was not observed. Reconstitution of Nu/+ splenocytes to Nu/Nu increased GM-CFC on day 6, and the increase was augmented by lentinan administration. These results suggest that mature T-cells participate in regulation of granulopoiesis in vivo and that lentinan augments granulopoiesis at least in part via mature T-cell populations.