Primordial follicles isolated from infant mouse ovaries by enzymatic disaggregation have been transferred to the ovaries of sterile adults. They were transferred after being recombined and incubated in a plasma clot which was inserted either into ovaries which had been sterilized by X-irradiation or into vacant periovarian capsules immediately after ovariectomy. Irrespective of the type of operation, these grafts underwent morphogenesis to form replicas of the normal ovary, which were generally indistinguishable in respect of form, histological appearance and function. The numbers of follicles in these so-called transgerminative ovaries were highly variable, but all stages of follicular maturation up to Graafian stage were represented in most of them, while grafts that had been X-irradiated before transfer were sterile. Ovaries forming from fertile grafts were capable of spontaneous ovulation and the majority of animals carrying them were sexually receptive to males. Mating often resulted in pregnancies and those delivering at full-term produced normal offspring.