The registration of biosimilars requires comparison studies to reference products to guarantee their safety, purity, efficacy, and potency. In this study, we demonstrate the similarity of a filgrastim produced by Eurofarma (Fiprima) and one produced by Amgen Inc. (commercialized by Hoffman-La Roche Ltd, Granulokine) in terms of drug identity, structure, purity, and bioactivity. The methods used to compare both products were the following: peptide mapping, bidimensional electrophoresis, reduced and nonreduced polyacrylamide electrophoresis, Western blotting, reverse-phase high-performance liquid chromatog., size-exclusion high-performance liquid chromatog., far and near CD, fluorescence emission, X-ray crystallog., liquid chromatog.-tandem mass spectrometry, matrix-assisted laser desorption/ ionization-time of flight, receptor binding, and potency by in vitro cell proliferation. Biosimilarity to Granulokine was demonstrated in terms of identity, structure, purity, and bioactivity.