This study reports a novel immunopotentiating effect of a lectin (FFrL) extracted from the mushroom Fomitella fraxinea on poultry cell-mediated immunity and poultry coccidiosis. We describe the extraction of FFrL, its in vitro mitogenic activity and in vivo protection against an oral challenge infection with Eimeria acervulina. When tested on several cell types, crude FFrL agglutinated mouse erythrocytes and thymocytes and various other cells including murine and human cell lines. However, crude FFrL did not agglutinate human erythrocytes. Crude FFrL showed a potent mitogenic activity on chicken splenic lymphocytes, and at lower concentrations it exerted stronger mitogenic activity than Concanavalin A, a well-known potent mitogen for lymphocytes. Further, FFrL significantly induced (P < 0.05) nitric oxide secretion in HD11 cells and suppressed (P < 0.05) RP9 tumor cell growth in a dose-dependent fashion. When injected into 18-d-old chicken embryos followed by a posthatch oral E. acervulina challenge infection, FFrL treatment significantly protected chickens against weight loss associated with coccidiosis (P < 0.05). Injecting embryos with FFrL also resulted in significant reduction in oocyst shedding as compared with the control saline-injected birds (P < 0.05). The results of this study demonstrate that FFrL can be an effective growth promoting and immunostimulating agent in poultry during coccidiosis.