Activation of the alternative pathway of complement (APC) and the Limulus factor G cascede by an alkali-soluble glucan, OL-2, isolated from a crude drug "leiwan" (Omphalia lapidescens) and its Smith-type degradation products were examined using human whole complement and Limulus lysate. OL-2 is a (1→3)-β-D-glucan having two branches at every third main chain glucosyl unit at each C-6 position (degree of branching (DB) : 2/3), and DB of the derivatives used in this study were OL-2-I (1/3), OL-2-II (1/2), OL-2-III (1/24) and OR-OL-2c (no branch : 0/1). Activation of APC by these glucans was dependent on incubation time and concentraiton. Under optimal conditions, OL-2-I, OL-2-II and OL-2-III showed stronger activation of APC than did the origical OL-2, and OR-OL-2c showed relatively weak activity. The structure-activity relationship of the activation of the factor G cascade was similar to that of APC. Considering the fact that the less branched derivatives of OL-2 also showed significantly stronger antitumor activity than OL-2, too much branching would suppress the recognition of (1→3)-β-D-glucan by the host-self defecse mechanisms.