Summary:A number of studies have suggested that prior chemotherapy correlates negatively with the efficiency of hematopoietic stem cell mobilization. However, little data exist with regard to the relative effects of the specific chemotherapeutic drug classes. We retrospectively reviewed the records of 201 consecutive patients with nonmyeloid malignancies undergoing CD34 þ cell mobilization with chemotherapy þ granulocyte colony-stimulating factor (G-CSF). The number of prior chemotherapy courses correlated negatively with the peripheral CD34 þ cell concentration (pCD34) on the first day of collection (Po0.001). No significant correlation was found for age, gender, tumor primary, mobilization chemotherapy regimen, disease status, marrow involvement, prior radiation therapy, or dose and timing of G-CSF administration. When the number of courses of individual classes of chemotherapeutic agents was correlated with pCD34, only exposures to platinum compounds (P ¼ 0.001) and alkylating agents (P ¼ 0.01) were found to be independent negative predictive factors for pCD34. Within classes, DNA crosslinking agents and etoposide appeared possibly more damaging than DNA methylating agents and doxorubicin, respectively. None of the drug classes showed evidence of recovery. We conclude that exposure to chemotherapy, especially platinum compounds and alkylating agents, should be minimized prior to mobilization.