SCF and FGF-2 stimulate BrdU incorporation in vitro. Treatment of normoxic cultures for 24 hours with SCF increased both the number of cells incorporating BrdU and the number of viable cells measured by MTT absorbance (a). FGF-2 also increased BrdU incorporation in these cultures (b), but the effects of maximally effective concentrations (10 ng/ml) of SCF and FGF-2 were not additive (c). Ab’s against SCF and FGF-2 each reduced the component of BrdU labeling stimulated by unfractionated HCM (all) by approximately 40%, but these effects were not additive (d). Anti-SCF and the combination of anti-SCF plus anti–FGF-2 (but not anti–FGF-2 alone) reduced BrdU labeling stimulated by the 30- to 50-kDa fraction of HCM by approximately 75%, consistent with the preferential localization of immunoreactive SCF to this fraction on Western blots. Anti–FGF-2 and the combination of anti–FGF-2 plus anti-SCF (but not anti-SCF alone) reduced BrdU labeling stimulated by the 10- to 30-kDa fraction of HCM by approximately 70%, consistent with the Mr value of 17.2 kDa for FGF-2. Data are mean ± SEM, n = 3, or representative blots from three experiments (inset to d). Asterisks in d indicate that the percentage of inhibition of BrdU labeling by a given Ab or combination of Ab’s is significantly different in that HCM fraction than in unfractionated HCM (P < 0.05; ANOVA and post hoc Student-Newman-Keuls tests).