This study investigates the effects of BP-53, the acid-stable IGF-binding subunit of the circulating 150 kDa IGF-binding protein complex, on IGF-I-stimulated thymidine incorporation by neonatal human skin fibroblasts. When cells were incubated for 24 h with IGF-I in serum-free medium, and thymidine incorporation studied over the final 4-h period, maximal stimulation (4- to 7-fold) was seen with 30 ng/ml IGF-I, with a half-maximal effect at 6.8±1.2 ng/ml (SD, n=4). Co-incubation of IGF-I with increasing concentrations of pure BP-53 caused dose-dependent inhibition of IGF-I-stimulated thymidine incorporation, which was complete when IGF-I and BP-53 were equimolar. In contrast, preincubation of cells with BP-53 for 8–48 h before adding IGF-I resulted in a potentiation of the subsequent IGF-I effect. The potentiation was maximal (2-fold) at a BP-53 concentration of 150 ng/ml, and appeared to act by increasing the maximal rate of thymidine incorporation rather than the sensitivity of this process to IGF-I. Since neonatal fibroblasts produce a protein which is identical to BP-53 in size and immunoreactivity, these results suggest an autocrine role for BP-53 in regulating fibroblast responsiveness to IGF-I.