A 15-kDa lectin was isolated from the edible mushroom Kurokawa by affinity chromatography using N,N'-diacetylehitobiose-Sepharose 4B. The results of microsequencing analysis indicated that the lectin has a partial amino acid sequence similar to the mushroom lectin, Agaricus bisporus agglutinin (ABA). We found that the Kurokawa lectin inhibited proliferation of human monoblastic leukemia U937 cells dose-dependently. Several lines of evidence indicated that this inhibition was due to its apoptosis induction. We observed that the lectin induced apoptotic bodies formation, chromatin condensation, and DNA ladder formation, features of apoptosis. The DNA ladder formation was inhibited by a general inhibitor of caspases, which are known to play essential roles in apoptosis. In contrast. ABA did not have cell growth-inhibiting or apoptosis-inducing activities. Thus, the Kurokawa lectin is the first mushroom lectin with apoptosis-inducing activity.