The heterogeneity of human transferrin results from (i) differences in iron content, (ii) genetic polymorphism and (iii) differences in the carbohydrate moiety. This article primarily deals with the last phenomenon, the microheterogeneity of human transferrin. Owing to the comparatively simple carbohydrate structure of human transferrin and the high resolving power of isoelectric focusing in immobilized pH gradients, microheterogeneous forms of transferrin can be separated. Differences between samples can be quantitated by crossed immunoelectrophoresis. Examples of the differences between the microheterogeneity patterns of transferrin in several biological fluids and the changes that can be observed in diseases such as rheumatoid arthritis, idiopathic hemochromatosis and Kahler's disease are presented. Special attention has been focused on changes occurring during pregnancy.