A D-galactose-specific lectin, MW = 40 kDa, had been purified from pupae of Musca domestica (MPL). MPL significantly promoted the proliferation of B cells and enhanced the production of IL-12 in a dose-dependent manner. MPL stimulated I kappa B-alpha degradation, NF-kappa B translocation and ERK1/2 phosphorylation which played an upstream role for NF-kappa B in MPL-induced B cells. Moreover, MPL regulated cell proliferation and induced IL-12 production through ERK1/2-NF-kappa B signaling pathway.