Two groups of mouse preantral follicles with diameters of 125-150 and 151-175 μm were cultured individually for 6 days in a medium supplemented with FSH and fetal calf serum to determine their in vitro growth characteristics. Their oocyte capacity for maturation and development to the blastocyst stage following in vitro fertilization was also assessed. Antral formation rate at the end of culture was higher in the follicles of 151-175 μm (89%) than 125-150 μm (76%). The timing of antrum formation was different between the two follicle categories: most 151-175 μm follicles formed antra earlier than 125-150 μm follicles (days 4 and 5 vs. 5 and 6). However, follicle diameters at the time of antrum formation were the same regardless of the initial size and the culture period. Maturation rates of the oocytes derived from both categories of in vitro grown follicles (70 and 62%) were not different from those of oocytes from in vivo grown follicles (74%). The in vitro derived oocytes, however, showed less cleavage (30 and 35%) than the in vivo derived oocytes (89%). Although the oocytes from both follicle categories developed to the morula stage after in vitro fertilization, blastocysts were only obtained from oocytes derived from the 151-175 μm category. These results demonstrate that an individual follicle culture system using a medium with FSH and fetal calf serum supports in vitro growth of mouse preantral follicles with diameters of 151-175 μm to the preovulatory stage, and that their oocytes have the capability to develop to the blastocyst stage.