Approximately 70% of patients with prostate cancer develop bone metastases in the advanced state of the disease. In the present study, we sought to test the hypothesis that prostatic cancer cells produce factors that inhibit the mineralisation process in vitro, decreasing the content of type I collagen in rat fetal calvaria osteoblasts. We investigated the capacity of conditioned media (CM) from the human prostatic tumour cell line PC-3 to inhibit the expression of the differentiation programme on osteoblasts in culture, with a primary focus on type I collagen synthesis and degradation. Our results show that PC-3 CM inhibits collagen synthesis and stimulates the production of interstitial collagenase from osteoblasts. A consequential decrease in the content of immunoreactive type I collagen was observed. We have previously demonstrated that PC-3 CM blocks osteoblast differentiation in culture. We propose that under the effect of factors present in PC-3 CM, osteoblastic cells retain the undifferentiated phenotype.