Insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and IGFBP proteases are important in ovarian function. IGFs stimulate granulosa steroidogenesis, an effect that is inhibited by IGFBP-4 and augmented by IGFBP-4 proteolysis. We have recently identified the IGFBP-4 protease in human ovarian follicular fluid (FF) as pregnancy-associated plasma protein-A (PAPP-A). In the current study, we identify the IGFBP-4 protease secreted by cultured human ovarian granulosa cells as PAPP-A, based on specific immunoinhibition and immunodepletion of the IGFBP-4 protease activity with PAPP-A polyclonal antibodies and immunorecognition by PAPP-A monoclonal antibodies in ELISA. PAPP-A was barely detectable in conditioned media (CM) from granulosa derived from /=9 mm, coincident with dominant follicle selection, and by luteinizing granulosa. PAPP-A levels in CM from the latter did not change in response to IGF-II or hCG (100 ng/mL). A naturally occurring inhibitor of PAPP-A, proform of eosinophil major basic protein (proMBP), was detected by ELISA in estrogen-dominant follicular fluid FF, but not in CM from granulosa or luteinizing granulosa cells treated with IGF-II (0-200 ng/mL), FSH (0-100 ng/mL) or hCG (0-100 ng/mL), suggesting an alternative source (other than granulosa) for proMBP, compared to PAPP-A. The data demonstrate granulosa cells as a source of PAPP-A in human ovary and suggest that PAPP-A is a marker of ovarian follicle selection and corpus luteum formation. In addition the data suggest complex regulation of this system in human ovary.