The efficacy of mobilizing peripheral blood progenitor cells (PBPC) with continuous intravenous (c.i.v.) administration of rhG-CSF was randomly compared to subcutaneous (s.c.) administration, in 15 normal donors in each arm of the study for 6 days. The percentage and absolute numbers of CD34+ cells in the c.i.v. and s.c. groups increased maximally at day 3 and 5, respectively, when compared with the steady-state (day 0) level. Peak CD34+ cell levels were achieved on day 3 in the c.i.v. group, with more rapid results than in the s.c. group (49.3/microl vs 35.9/microl, P=0.043). Plasma rhG-CSF levels declined progressively during mobilization in each group as the WBC increased. The serum level of rhG-CSF did not correlate with CD34+ cell counts in the peripheral blood. Toxicity profiles in the c.i.v. and s.c. groups were similar. Each regimen was effective in successfully mobilizing the target CD34 cell number.