Bovine follicular fluid was aspirated from follicles of 2-20 mm in diameter, charcoal-treated to remove steroids and then separated into low and high molecular mass fractions. The low molecular mass ( 10 mm) follicles. The inhibitory effect of peak 4 was not due to a toxic effect on cells. Administration of peak 4 to rats did not affect liver or kidney masses but did decrease uterine (25%, P < 0.01) and ovarian (35%, P < 0.01) masses. Peak 4 also caused a reduction in the number of large follicles (65%, P < 0.01) but increased the number of small follicles (55%, P< 0.01). We have named the inhibitory factor associated with peak 4, granulosa cell-inhibitory factor (GCIF). The results presented suggest that GCIF may be a factor secreted by dominant follicles that inhibits the development of: subordinate follicles.