Forty-seven peripheral blood stem cell (PBSC) collections were carried out on patients mobilized with chemotherapy and 63 on patients mobilized with chemotherapy plus G-CSF (Filgrastim), using the Fresenius AS104 cell separator and a novel automated PBSC collection protocol. As expected, cell yields were significantly higher in the series mobilized using chemotherapy plus G-CSF. The low platelet and red blood cell contamination permitted freezing of the apheresis product without further manipulation, other than plasma removal in both series. In patients mobilized with chemotherapy we obtained a MNC and a hemopoietic progenitor (CFU-GM, BFU-e, and CD34+ cells) collection efficiency comparable or superior to those reported by Bender (1992) with the Baxter CS3000 Plus after mobilization with cyclophosphamide. A significant decrease in MNC, BFU-e, and CD34+ cell collection efficiency was found in patients mobilized with chemotherapy plus G-CSF compared to those obtained in patients mobilized with chemotherapy alone. Ten patients achieved a prompt and stable engraftment after high dose chemotherapy and the infusion of cryopreserved PBSC collected using this protocol. Studies are in progress in order to improve MNC and hemopoietic progenitor collection efficiency in patients mobilized with G-CSF to obtain a graft in no more than one or two procedures.