High-dose conditioning regimens followed by autologous peripheral blood stem cell rescue are frequently used for the treatment of solid tumors and hematological malignancies. In 24 patients up to four peripheral stem cell collections (PBSC) were performed after priming with various chemotherapies and G-CSF (300 micrograms s.c. per day). In 16 patients (group A) more than 2 x 10(6) CD 34 positive cells per kg bodyweight could be collected; fewer were harvested in the remaining eight patients (group B). The amount of collected CD 34 positive cells correlated with the median number of these cells in the peripheral blood at the start of PBSC. The two groups differed both in recovery time after priming-induced cytopenia (4 vs 6 days from nadir) and in the number of WBC (21 x 10(6) mL-1 vs 6.1 x 10(6) mL-1) and platelets (133 x 10(6) mL-1 vs 58 x 10(6) mL-1) reached at first day of PBSC. No difference between the two groups was seen according to age, duration of disease or disease status. However, the intensity of prior treatment was significantly greater in group B than in group A. These observations indicate that the toxicity of previous chemotherapy is the most important factor for the mobilization of sufficient CD 34 positive cells into the peripheral blood.