Lectin activity, agglutinating sheep erythrocytes, was associated with parasporal inclusion proteins from a Lepidoptera-specific isolate of Bacillus thuringiensis serovar galleriae (1-15ab). The activity was generated when parasporal inclusions were solubilized in an alkaline condition. Proteolytic processing was not required for generation of the lectin activity; the activity level was not affected by the presence/absence of the three proteases (trypsin, chymotrypsin, and proteinase K). SDS-PAGE analysis revealed that (1) alkali-solubilized parasporal inclusion proteins consisted of two major components of 130 kDa and 65 kDa, and (2) proteinase K treatment of alkali-solubilized proteins yielded a single major protein of 60 kDa. Lectin activity of our isolate was strongly inhibited by preincubation with D-mannose, but not with the six other monosaccharides: D-galactose, D-glucose, L-fucose, N-acetyl-D-glucosamine, N-acetyl-D-galactosamine,and N-acetylneuraminic acid. In contrast, D-mannose did not inhibit the in vivo larvicidal activity of the proteins against the silkworm, Bombyx mori.