We report cell kinetics, yields and donation experiences of 20 demographically matched allogeneic peripheral blood stem cell (PBSC) donors who were prospectively assigned to undergo either a single 25 l or two consecutive daily 15 l (15 l x 2) apheresis procedures. Procedures were performed using prophylactic intravenous calcium administration after standard granulocyte colony-stimulating factor (GCSF) mobilization (10 microg/kg/d). Central line placements (two each), initial CD34 cell counts (0.077 vs 0.078 x 10(9)/l) and yields (7.9 vs 8.1 x 10(8) CD34 cells) were similar in the two groups; however, 25 l donors spent significantly less time both in the clinic (7.5 vs 10.8 h) and with central venous catheters in place (8.5 vs 29.5 h) than 15 l x 2 donors. End-procedure platelet counts were below 100 x 10(9)/l in one out of 10 25 l donors versus five out of 10 in 15 l x 2 donors (41%vs 53% mean decrease in platelet counts, P = 0.02). PBSC collection efficiency increased by 37% after 15 l of the 25-l volume had been processed, compared with no significant change during 15 l x 2 procedures. Results similar to these prospective findings were also observed in CD34 yields, symptoms and platelet counts in additional 25 l and 15 l procedures performed during the same period and evaluated retrospectively. This study indicates that a single 25-l apheresis procedure results in similar yields and symptoms, but less donor thrombocytopenia and inconvenience than two consecutive daily 15-l procedures.