Abstract Very pure preparations of human serum albumin prepared by preparative electrophoresis or ethanol fractionation were subjected to Chromatographic analysis on modified cellulose and individual Chromatographic fractions were analysed by immunoelectrophoresis or electrophoresis on agar gel. It was demonstrated that albumin can be separated into two distinct fractions, the larger of which can probably be subdivided further. In some Chromatographic fractions of the two preparations investigated, the albumin present behaved normally during electrophoresis on agar gel, whereas during immunoelectrophoresis an incomplete precipitation reaction was obtained indicated by the formation of a very weak, diffuse precipitate. It can be assumed that albumin contains molecules with different antigenic structures.