In vivo responses of xenografts ALL-7 and ALL-17 to vincristine. Mice were inoculated with ALL-7 (A) or ALL-17 (B), monitored for engraftment, and treated with vincristine (solid lines) or saline control (dotted lines) as described in “Materials and methods.” During and following treatment, the leukemic burden was monitored by estimating the proportion of human CD45⁺ cells in murine peripheral blood. Each line is representative of a single mouse. Whereas saline-treated control xenografts grew at equivalent rates (A-B), ALL-7 reappeared in the peripheral blood before the final vincristine treatment (A), and ALL-17 took approximately 7 weeks from the initiation of treatment to progress (B). Data presented are from a representative experiment. Arrows indicate vincristine or saline treatment times.