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In cultured untreated ovaries, oocytes from primordial follicles were considerably larger (P < 0.05) than those of primordial follicles in preculture tissues, whereas the mean diameters of oocytes from follicles at all other stages of development were unaffected by culture (Table 2). The spontaneous enlargement of oocytes from primordial follicles was not antagonized by KITL-neutralizing antibody (Table 2), suggesting that some unknown aspect of culture other than endogenous KITL induced the observed growth. Supplementation of culture media with recombinant mouse KITL promoted a significant (P < 0.05) increase in the mean diameter of oocytes from mouse primordial and early primary follicles compared with those in untreated cultured ovaries (Table 2), but only with the higher dose. Furthermore, the growth-promoting effects of 150 ng/ml recombinant mouse KITL on primordial and early primary follicle oocytes were completely inhibited by the addition of KITL-neutralizing antibody (Table 2). In contrast, recombinant mouse KITL, either alone or in combination with neutralizing antibody, had little effect on the oocytes of follicles at the later stages of development. In general, ovaries treated with 50 ng/ml or 150 ng/ml recombinant mouse KITL contained primary, growing primary, and preantral follicles with oocyte diameters similar to their counterparts in untreated cultured ovaries. The exception to this trend was growing primary follicles, for which the mean oocyte diameter was significantly (P < 0.05) larger in ovaries treated with 50 ng/ml recombinant mouse KITL compared with untreated ovaries. These data support a role for KITL in promoting murine oocyte growth during the earliest stages of follicle development, whereas other growth stimuli may become more influential in primary, growing primary, and preantral follicles.