Human non-autocrine neuroblastoma cells SK-N-SH and LF require serum for proliferation in vitro. We wished to determine the role of serum-borne insulin-like growth factor I (IGF-I) as mitogen for these cells. Introduction of the monoclonal antibody alpha-IR3 against human IGF-I receptor reduced proliferation in the presence of fetal bovine serum (FBS). IGF-I (5 nM) was as effective as FBS (10%) in stimulating proliferation. Porcine insulin mimicked the effects of IGF-I, but at a 1000-fold higher concentration. The antibody alpha-IR3 reduced growth stimulated by IGF-I more effectively than growth stimulated by insulin. Thus, proliferation of human non-autocrine neuroblastoma cells can be effectively manipulated by exogenous IGF-I.