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Comparisons of oocyte sizes between untreated and recombinant mouse KITL-treated cultured explants were made in order to assess the effect of exogenous recombinant mouse KITL on oocyte growth in vitro. Recombinant mouse KITL was shown to have a major effect on the size of oocytes at the earliest stages of development (Table 1). The mean diameter of primordial follicle oocytes increased significantly (P < 0.05), from 31.7 6 0.2 im in untreated cultured explants, to 34.6 6 0.3 im in explants treated with 50 ng/ml recombinant mouse KITL, to a final diameter of 36.3 6 0.1 im in 150 ng/ml recombinant mouse KITL-treated explants (Table 1). In addition, KITL-neutralizing antibody antagonized (P < 0.05) the increase in oocyte diameter caused by 150 ng/ml recombinant mouse KITL (Table 1). Similar to primordial follicles, the mean oocyte diameters of early primary, primary, and growing primary follicles increased with exposure to rising doses of recombinant mouse KITL (Table 1). Surprisingly, the stimulatory effects of recombinant mouse KITL at these stages of development were not mitigated by the addition of neutralizing antibody to the culture media (Table 1). Why antiserum should partially inhibit the effects of recombinant mouse KITL in primordial follicles but not at later developmental stages is unclear.