Fifteen consecutive patients with multiple myeloma (MM) scheduled for peripheral blood progenitor cell (PBPC) transplantation, were randomly selected to receive cyclophosphamide (CY) (4 g/m2) alone (group I) or associated with recombinant human granulocyte-macrophage colony stimulating factor (rhGM-CSF) (5 micrograms/kg/day) (group II). The mean time of neutropenia after CY administration was 9.8 +/- 4.3 days in group I and 6.4 +/- 1.2 days in group II (P = 0.0228). One hundred and eight aphereses were performed (7.1 +/- 1.8 aphereses per patient in group I and 6.4 +/- 2.8 aphereses per patient in group II). rhGM-CSF administration after CY allowed a higher collection of CD34+ cells in apheresis products (1.42 +/- 1.68 x 10(6) CD34+ cells/kg) in comparison to without factor administration (0.47 +/- 0.52 x 10(6) CD34+ cells/kg) (P = 0.0165). The mean number of cells infused per patient was 6.56 +/- 4.02 x 10(8) MNC/kg and 7.64 +/- 3.00 x 10(4) CFU-GM/kg in group I and 6.25 +/- 4.03 x 10(8) MNC/kg and 8.16 +/- 9.73 x 10(4) CFU-GM/kg in group II. The mean time to recover 0.5 x 10(9) granulocytes/I, 20 and 50 x 10(9) platelets/I in peripheral blood (PB) was 17.2 +/- 7.4, 13.4 +/- 3.7 and 16.5 +/- 6.9 days respectively, in group I and 13.3 +/- 1.7, 11.6 +/- 1.6 and 15 +/- 6.3 days, in group II. rhGM-CSF administration after CY treatment for PBPC mobilization in MM patients reduces the neutropenic period after CY and enhances apheresis CD34+ cell collection.