We report the preliminary results of a study exploring the possibility of collecting circulating progenitor cells (PBSC) with a protocol based on the administration of single doses (4 g/m2) of cyclophosphamide and G-CSF (5 or 10-micrograms/kg) in 9 patients with non Hodgkin's lymphoma. The peak level of CD34+ cells occurred after a median of 10 days (range 8-11), generally coinciding with the median peak level of CFU-GM, with a mean 31.27 fold increase above basal levels. 3 (range 2-5) leukaphereses were required to harvest a median number of 25.1 x 10(4)/kg (8-105) CFU-GM and of 9.4 x 10(6)/kg (1.2-25) CD34+ cells. No difference was recorded between 5 and 10 micrograms/kg of G-CSF in terms of PBSC yield. In transplanted patients, a strong correlation was found between CD34+ cells infused/kg and platelet recovery (r = -0.8, p = 0.002). No toxicity was observed and apheretic procedures were regularly performed outpatiently. Our conclusion is that this protocol is particularly suitable for an outpatient treatment/collection program.