Context: The signals initiating growth of primordial follicles are unknown. Bone morphogenetic protein 15 (BMP15) and growth differentiating factor 9 (GDF9) are promising candidates. Objective: The objective of the study was to evaluate for the first time the effects of human recombinant BMP15 and human recombinant GDF9 on the in vitro development of human primordial follicles. Design and Setting: This was a controlled culture study performed in a major tertiary university-affiliated medical center. Materials: Materials included ovarian tissue from 17 girls/women and three aborted human fetuses. Intervention: There were no interventions. Main Outcome Measure: Histological and immunohistochemical (proliferating cell nuclear antigen, BMP15, and GDF9) studies and an endocrine assay of 17β-estradiol were conducted. Results: In the samples from girls/women, the number of developing follicles was greater with GDF9 or BMP15 alone than with no BMP15 or GDF9. Higher 17β-estradiol secretion was noted after treatment with GDF9 than with BMP15 or with GDF9+anti-GDF9. The number of atretic follicles was greater with BMP15 than with GDF9. Proliferating cell nuclear antigen expression was greater with the higher dose of both growth factors than the lower dose. Expression of BMP15 and GDF9 was identified in samples cultured without BMP15 or GDF9. Results for the fetal follicles yielded no distinguishable pattern. Conclusions: Although both BMP15 and GDF9 promoted activation of human primordial follicles from girls/women (but not human fetuses) in a dose-dependent manner, GDF9 seems more beneficial.