Purpose To determine whether Activin A affects the activation and survival of human primordial follicles in vitro. Methods Ovarian cortical biopsies from eight women undergoing elective caesarean sections or benign gynaecological procedures were taken and cut into small pieces (1-3 mm 3 ), cultured in serum-free medium for 7 days with/without human recombinant Activin A at a concentration of either 50 or 100 ng/ml. Ovarian tissue were analysed by histology for follicle viability, development and density. Result(s) Significant activation of primordial follicles within cultured cortical tissue was observed after 7 days in control medium. However, medium supplemented with Activin A at 50 ng/ml resulted in significant inhibition of follicular activation. Increasing the concentration of Activin A to 100 ng/ml reversed the inhibitory effect. The effect of Activin A appeared to be specific to activation of non-growing (primordial) follicles into the growing population since no significant differences in follicle viability was observed between treatment groups. Conclusion(s) Activin A at a concentration of 50 ng/ml can inhibit the spontaneous activation of human primordial follicles in vitro indicating that this may be a component of the signalling mechanisms that maintain follicular quiescence.