Antibody for (1-->6)-branched (1-->3)-beta-D-glucan was prepared using rodents. An antitumor (1-->6)-beta-monoglucosyl branched (1-->3)-beta-D-glucan (GRN: grifolan) was conjugated with bovine serum albumin and used as an immunogen. The antibody titer in serum was determined by ELISA using biotin-conjugated GRN. Administration of the antigen raised the antibody titer only in the rabbit, with mouse and rat showing no significant antibody titer for the glucan. The antigen specificity of the anti-GRN antibody was determined by competitive ELISA. The rabbit anti-GRN antibody bound to structurally related antitumor (1-->6)-branched (1-->3)-beta-D-glucans such as lentinan, schizophyllan and SSG, whereas it did not react with linear (1-->3)-beta-D-glucan, curdlan or GRN-derivatives obtained by periodate-oxidation and Smith degradation. These facts strongly suggest that the hapten site of the antibody was the monoglucosyl branched moiety of (1-->3)-beta-D-glucan. These results indicate that this antibody would be a useful probe for the detection of (1-->6)-branched antitumor glucans administered to the host.