Alpha IR-3, a monoclonal antibody to the insulin-like growth factor I receptor which blocks insulin-like growth factor I binding and inhibits its activity, inhibits the binding of 125I-insulin-like growth factor I to MCF-7 cells (an estrogen dependent human breast carcinoma cell line) with an IC-50 of approximately 100 ng/ml. It also inhibits the growth of MCF-7 cells cultured in 5% calf serum with approximately the same IC-50. Inhibition of growth occurs both when cells are cultured in the presence and absence of estrogen and is more pronounced when cells are grown at a low density. These findings demonstrate a requirement for insulin-like growth factor I for optimal growth of MCF-7 cells and suggest that it is an autocrine growth factor in these cells. © 1987.