Midkine (MK) is a heparin-binding growth factor specified by a retinoic acid responsive gene. A rabbit was immunized against recombinant MK produced in L cells, and the resulting antibody was affinity purified using MK-glutathione S-transferase fusion protein as a ligand. The MK-specific antibody was used to investigate the function and distribution of MK. MK of the same size as the recombinant MK produced in L cells (13 kDa) was strongly detected in a 13-day rat embryo. Weak expression was observed in the brains of a 19-day embryo, neonates, and adults. In the 13-day mouse embryos, high levels of MK were detected on the surfaces of brain cells, as well as in basement membranes and in the epithelial cells of the intestine, the jaw, and the rib. Nerve cells from the brains of 13-day or 19-day rat embryos extended neurites about twofold more efficiently on MK-coated dishes than on poly-L-lysine-coated dishes. Furthermore, anti-MK antibody inhibited neurite extension not only on MK-coated dishes, but also on poly-L-lysine-coated dishes. These results suggest that MK is an endogenous neurite outgrowth factor involved in the development of the central nervous system. Anti-MK antibody was also found to inhibit growth of Wilms' tumor cells, which secreted MK into culture medium. Thus, overproduction of MK is involved in enhanced growth of Wilms' tumor cells.