Human chorionic gonadotropin (hCG) exhibits considerable heterogeneity when analysed using charge-based separation techniques. When analysed by capillary electrophoresis (CE), using a borate buffer system, six major and eight minor isoforms of hCG can be resolved. The aim of this study was to determine the basis for this separation. Following treatment with neuraminidase, an enzyme which removes sialic acid residues, the series of peaks observed with untreated material disappeared and were replaced by a set of faster migrating peaks. This observation is consistent with a reduction in negative charge, associated with the removal of sialic acids, and demonstrates that the hCG heterogeneity observed following CE is due primarily to variations in sialic acid content. As the degree of sialylation of hCG is correlated with its biological activity, CE may be used to assess qualitatively hormone potency. These data serve to highlight the potential of CE, to the biotechnology industry, for use in the quality assurance and control of hCG and other hormones produced by recombinant technologies.