Fold progenitor reduction in the absence (−) or presence (+) of preceding SCF administration for taxol, navelbine, and vinblastine. The fold progenitor reduction was determined by the number of progenitors per femur in control animals divided by the number of progenitors per femur in cytotoxic drug-treated animals. Note that for each drug, preceding SCF administration augments progenitor depletion threefold to sixfold. Using a two-tailed Student's t-test, these results are statistically significant for taxol at P < .05, for navelbine at P < .1, and for vinblastine at P < .005.