In vitro studies indicate that lenograstim (glycosylated G-CSF) is more potent than filgrastim (nonglycosylated G-CSF) on a weight for weight basis. However, such a difference has not yet been shown in vivo. The primary objective of this trial was to compare the efficacy of equivalent doses (microgram) of lenograstim and filgrastim in mobilizing CD34+ cells. Thirty-two healthy male volunteers, median age 27 yr (19-44 yr), were randomized to receive either lenograstim 10 micrograms/kg followed by filgrastim 10 micrograms/kg or vice versa with a washout period of a minimum 4 wk. Both drugs were administered as s.c. injections once daily for 5 d (d 1-5). CD34+ cells were mobilized with a similar kinetics, peaking at median d 6 (5-6) for both drugs. A significant difference in favour of lenograstim was shown for peak number of CD34+ cells/microliter blood (104 +/- 38 vs. 82 +/- 35, mean +/- 1 SD, p < 0.0001, paired t-test, n = 30) and number of CFU-GM/microliter blood at d 6 (14.6 +/- 8.4 vs. 10.2 +/- 4.6, p < 0.0001), respectively. There was no difference in the d 6 number of CD3+ cells. Both drugs were generally well tolerated and did not differ with respect to number of adverse events. In conclusion, lenograstim 10 micrograms/kg/d mobilizes PBPC more efficiently than the identical dose of filgrastim, indicating a difference in in vivo potency between the two G-CSFs.