SUMMARY Equine granulosa and other cell types have been cultured for up to 70 days in a medium containing 15% horse serum, 30% medium '199' and 55% Hanks's solution. Granulosa cells harvested from large (3–6 cm. in diameter) vascular follicles of mares in oestrus grew in epithelioid colonies. Cell division and hypertrophy lasted for 7 days with the greatest amount of hypertrophy occurring during the first 3–4 days in culture. Within 3 days the cytoplasm increased in size and acquired eosinophilic granules and lipid droplets; nuclei acquired distinct chromatin material and nucleoli. Cells harvested from small follicles grew in fibroblastic stellate colonies and underwent hypertrophy and hyperplasia but did not show any of the cytoplasmic characteristics of the cultures of granulosa cells obtained from the large follicles. Heterogeneous fibroblastic cells grew from explants of stromal and thecal tissue from days 5 to 20 of culture.