Female rabbits were actively immunized either against bovine serum albumin (BSA) or oestradiol-6-oxime BSA. When the antibody titres were elevated they were sacrificed and various parameters measured. Uterine weights were significantly lower, and a large number of haemorrhagic and some cystic follicles were present in the ovaries of the oestradiol immunized group. Increased serum gonadotrophins, testosterone, oestradiol, and progesterone levels as well as greater serum binding of oestradiol were also noted. Examination of the response of isolated follicles to gonadotrophin treatment in vitro indicated that oestradiol immunization decreased the ability of isolated follicles to form androgen and oestrogen following gonadotrophin treatment; however, progesterone levels were high regardless of the presence or absence of gonadotrophins. These data suggest that oestradiol plays an important role in follicular development.