CFU become increasingly resistant to BG and BCNU after repetitive drug administration.

Bone marrow cells were treated with 20 μmol/L BG and 0 to 80 μmol/L BCNU, then plated in methylcellulose. CFU growth was scored, and resistance curves were generated. Error bars represent SD of mean percentage survival. ▪, 3 cycles of BG and BCNU; ▴, 2 cycles; •, 1 cycle; dashed line; open box, 0 cycles; dotted line, normal mice.