The biological potency of 2 recombinant human granulocyte colony-stimulating factors (rG-CSF) formulated for clinical use (lenograstim and filgrastim) together with a nonmarketedEscherichia coli-derived rG-CSF were compared by duplicate neutrophil colony assays in 3 normal human marrows. Serial doubling dilutions of each rG-CSF (128 to 0.008 ng/ml) were used. Neutrophil colonies were blindly scored at day 14. Lenograstim stimulated neutrophil colony formation at doses 16 times lower than that of the 2 nonglycosylated rG-CSFs and was twice as potent as filgrastim at maximal colony stimulation. Qualitative differences in colony size were also noted. Both nonglycosylated rG-CSFs (filgrastim and research-use rG-CSF) gave inferior qualitative and quantitative results, suggesting that glycosylation gives lenograstim a potency advantage.