Bone morphogenetic protein 15 (BMP15) and growth and differentiation factor 9 (GDF9) are secreted by the mammalian oocyte and are essential for ovarian follicular development, ovulation, and fertility. However, the secreted forms of the BMP15 and GDF9 proteins and the nature of cooperative molecular interactions between BMP15 and GDF9 previously reported have not been fully characterized. In this study, we found that recombinant mouse BMP15 and GDF9 are secreted as cleaved mature and proregion proteins, with BMP15 also secreted as uncleaved promature protein. Noncovalent interactions were identified between the mature and proregion proteins of each growth factor. Moreover, GDF9 mature protein was found to coimmunoprecipitate with the BMP15 proregion, suggestive of a heteromeric association between BMP15 and GDF9. Mouse GDF9 was found to exist mostly as a dimer of mature protein, in both the presence and absence of BMP15. In contrast, BMP15 formed mostly multimers of proregion and mature protein when combined with GDF9, providing further evidence for heteromeric interaction. Mouse BMP15 was found to act cooperatively with GDF9 in a rat granulosa cell thymidine incorporation bioassay and to signal through the BMPR2 and ACVR1B/TGFBR1/ACVR1C receptor-mediated pathways. Immunoneutralization experiments using GDF9 mature protein antibody indicated that these cooperative interactions are species specific. Additionally, immunoneutralization with proregion antibodies highlighted the involvement of the BMP15 proregion in BMP15/GDF9 cooperative interactions. Taken together, these findings support a novel hypothesis where the extracellular cooperative interactions of recombinant mouse BMP15 and GDF9 are multimeric, involving the proregion of BMP15, and may well be species specific..