Preceding SCF administration allows significantly reduced doses of taxol (A) and vinblastine (VBL; B) to achieve levels of progenitor depletion that are not reached with very high doses of drug alone. The upper panels show the total clonogenic progenitors per femur. CTL, control animals receiving no drug; SCF, animals receiving SCF alone; SCF→, preceding SCF administration as shown in Fig 1; 100% taxol, 60 mg/kg; 50% taxol, 30 mg/kg; 10% taxol, 6 mg/kg; 100% VBL, 10 mg/kg; 50% VBL, 5 mg/kg. The lower panels show the fold progenitor reduction as determined by dividing the number of progenitors per femur in animals treated with either no drug (CTL) or SCF alone (SCF) by the number of progenitors per femur in animals treated with drug alone or with preceding SCF administration. Note that preceding SCF administration allows the doses of both taxol and vinblastine to be reduced by more than half, while retaining a level of progenitor depletion that cannot be accomplished using maximal doses of drug alone. Error bars indicate standard deviations.