Isoelectric focusing of serum immunoglobulin G (IgG) revealed that the microheterogeneity, which is expressed in the isoelectric points (IEP), partly is caused by differences in the content of N-acetyl-neuraminic acid (NANA), partly by other effects, probably including deamidation. Two different types of Plasmocytoma-IgG, which differ in IEF-pattern, i.e. population distribution, in sensitivity towards neuraminidase and in carbohydrate content, are described. The contents of carbohydrates in these 2 IgG-types are up to 30% greater and 50% less, respectively, than that of normal IgG. A precise correlation was found between the total content of NANA in moles per mole IgG-monomer and the shift in band-IEP on hydrolysis catalyzed by neuraminidase. This may be used for a rapid estimate of NANA in IgG. The results do not permit a discrimination between an anabolic and a catabolic origin of the heterogeneity in the carbohydrate moiety of IgG.