The biological activities previously described for a crude extract derived from seeds of Artocarpus integrifolia (jack fruit) are shown in the present work to be assigned to two distinct fractions present in this extract. One fraction is the D-galactose binding lectin, jacalin, obtained by affinity purification on a D-galactose Sepharose column. The other fraction is a D-mannose-binding protein which we propose to call 'Artocarpin'. As is well documented, jacalin binds to human IgA1 and is a useful tool for the purification of this immunoglobulin. We show here that the remaining biological activities consisting of the proliferative response of mouse spleen cells and human peripheral blood mononuclear cells and polyclonal activation of human and mouse B cells for the secretion of immunoglobulin are mediated by artocarpin. Artocarpin is unique in its capacity to induce polyclonal activation of B cells in the absence of proliferation. BALB/c nu/nu spleen cells failed to proliferate which indicates that this lectin is a T cell-dependent B cell polyclonal activator.