Insulin and insulin-like growth factor I (IGF-I) are structurally related polypeptides that stimulate DNA synthesis and cellular proliferation, probably through a common pathway. Human arterial smooth muscle cells in culture demonstrated the presence of high-affinity receptors for both these hormones. Insulin and IGF-I both exhibited cross-reactivity to each other's receptors but with an affinity that is 100-fold less than for the homologous receptor. To examine more closely the receptor responsible for producing the growth effects, we used the polyclonal antibody against the insulin receptor, B2, and a monoclonal antibody to the IGF-I receptor, αIR3 . We studied the growth effects of insulin and IGF-I as measured by stimulation of c-myc, DNA synthesis, and cellular proliferation in the presence and absence of these antibodies. F(ab') fragments of the anti-insulin-receptor antibody at a concentration of 10 μg/ml were capable of displacing >90% of the bound insulin, thus establishing an effective insulin-receptor blockade. Under such blockade, insulin and IGF-I were both capable of doubling the amount of DNA synthesis and cell number in cultured human arterial smooth muscle cells. However, in the presence of a 1:2500 dilution of the monoclonal antibody αlR3 , which caused a 90% displacement of IGF-I bound to its receptor, both the insulin and IGF-I effects on stimulating DNA synthesis or cellular proliferation were inhibited by >90%. These findings demonstrate that the IGF-I receptor is the common pathway for the growth effects of both insulin and IGF-I. Induction of the protooncogene c-myc is recognized as an early response to many mitogenic stimuli. In human smooth muscle cells, insulin (1 × 10−7M) and IGF-I (1 × 10−9M) both cause a 5- to 10-fold increase in c-myc mRNA levels. Induction of c-myc could not be assessed under conditions of selective IGF-I-receptor blockade with the antibody αIR3, because, surprisingly, the antibody itself stimulated c-myc mRNA levels. This induction of c-myc with αIR3, which does not increase DNA synthesis or cellular proliferation, suggests that c-myc induction does not entirely correlate with the growth effects of these hormones.