In an attempt to improve hemopoietic recovery after autologous bone marrow transplantation (ABMT), a project of peripheral blood stem cells (PBSC) harvest was initiated. Thirty-six children awaiting ABMT underwent 126 PBSC harvests primed by a conventional scheduled chemotherapy course and rGMCSF 5 micrograms/kg per day/s.c. Ages ranged from 12 months to 19 years and weight from 9 to 84 kg. PBSC harvest was carried out using the Fenwall CS 3000 Plus with the small volume collection chamber at a maximum whole blood flow rate of 15-45 ml/min; total volume processed was 1,700-10,000 ml. Total nucleated cells per collection was 0.35-5.62 x 10(8) cells per kg, and the number of CD34+ cells, 0.23-1.1 x 10(6)/kg. The number of colony forming units-granulocyte macrophages (CFU-GM) varied in these heavily pretreated patients from 0 to 5.3 CFU-GM x 10(4)/kg per collection. Immunophenotyping of the cells collected was performed by double staining for CD34, CD33, CD15, CD71, Ia and CD56. Most of the CD34+ cells were found to be CD38+; some were CD33+ and some CD33-. Low coexpression of CD34+ CD71+ cells may correlate with the low proliferating capacity of PBSC as compared to the BM cells. To date 22 children have undergone transplantation using combined autologous PBSC and bone marrow. We conclude that PBSC harvest is a feasible and safe procedure even in small children, and can be successfully performed following scheduled chemotherapy and administration of growth factors, resulting in substantial yield, also in heavily pretreated patients. This procedure is recommended in responding high risk patients at the stage of minimal residual disease and may replace ABMT in the future.