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Deregulation of growth in malignant cells has been suggested to be the result of increased secretion by these cells, of autocrine growth factors; alternatively, this deregulation has been assumed to be related to loss of sensitivity by malignant cells to secreted inhibitory molecules. The results of the present publication lend new support to both hypotheses. We recently showed that human prostate adenocarcinoma cells (PC-3 cells) secreted insulin-like growth factor binding proteins (IGFBP) of 45, 34 and 25 kDa. From medium conditioned by dense cultures of PC-3 cells, we have now purified two IGFBPs of M(r) 45 kDa and 34 kDa. The N-amino terminal sequences were determined, and it was shown that they are IGFBP-3. IGFBP-34 appeared to be a deglycosylated form of IGFBP-45. The two IGFBPs had more affinity for IGF-II than for IGF-I. IGFBP-45 and IGFBP-34 were growth-inhibitory factors of chick embryo fibroblasts (CEF): they totally inhibited DNA synthesis stimulated by serum in CEF. Our results point to a clear difference between the effects of these IGFBPs upon growth of normal prostate cells and malignant PC-3 cells. At a concentration of 150 ng/ml, they inhibited growth of normal prostate cells even in the presence of 1 microgram/ml insulin. This suggests that such inhibition was not simply the result of a decrease by the IGFBP of stimulation induced by serum IGF or IGF secreted by the cells. At a concentration of 150 ng/ml, IGFBP did not modify the growth of PC-3 cells. In contrast, it stimulated growth of PC-3 cells when added at a concentration lower than 50 ng/ml (about 1 nM). Our results thus provide new insight concerning the regulation of growth in PC-3 cells.