Administration of filgrastim influences the proliferative kinetics of myeloid progenitor cells. Even after it is discontinued, increased levels of cycling of granulocyte precursors are sustained for approximately 4 days. Beginning chemotherapy during this period of enhanced marrow activity can cause great damage to late, and possibly early, progenitor cell pools. Peripheral blood samples were collected from two research study patients who were prospectively randomized to receive filgrastim by different schedules after chemotherapy. The mononuclear cell fraction was analyzed by clonogenic progenitor cell assay and flow cytometry. Ex vivo and clinical findings were correlated with filgrastim and chemotherapy administration times. Although quantitative recovery of circulating neutrophils occurred, a substantial decrease in peripheral blood progenitor cells was observed when chemotherapy was started 72 hours after cessation of filgrastim therapy. Neutrophil recovery alone is not a precise index of short-term marrow granulocyte progenitor status. Starting chemotherapy within 72 hours of filgrastim therapy is biologically, and possibly, clinically relevant.