ISOANTIGENS, thought to be important as histocompatibility antigens, have been identified on membranes of human cells by serological techniques1. In order to interpret serological studies of histocompatibility antigens in terms of an understanding of the molecular representation of gene information, the isoantigens must be obtained in a suitable state for detailed chemical analysis. Preparation of soluble isoantigen in relatively low molecular weight form would be an important step in this direction.