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In order to determine whether endogenous KITL had a role in promoting the growth of oocytes from primordial and early primary follicles that was observed during in vitro culture, neutralizing antibody to recombinant mouse KITL was added to the culture media. It was anticipated that if endogenous KITL were inducing growth in follicles maintained in vitro, then neutralization of the ligand would result in primordial and early primary follicle diameters comparable to those in preculture tissues. However, antibody added alone (i.e., in the absence of exogenous KITL) resulted in increased oocyte diameters (P < 0.05) in primordial, early primary, and primary follicles compared with those in untreated cultured explants (Table 1). The stimulatory effect was lost at the growing primary and preantral follicle stages. The mechanisms responsible for the induction of early follicle growth in the presence of antibody are unknown. Possibly, the neutralization of endogenous KITL permits other factors present in the culture media that affect follicle and oocyte growth to exert effects which would normally be masked by the presence of KITL.