A necessary step in the induction of unresponsiveness to transplantation antigens is the preparation and presentation of tissue antigens in a suitable form. A method is presented for obtaining rat histocompatibility antigen in a soluble, “haptenic” form suitable for coupling to a tolerogenic carrier. Lewis and ACI spleen cell membranes are solubilized with deoxycholate and purified by Lens culinaris affinity chromatography. The ultracentrifuged, solubilized preparation of rat histocompatibility (HC) antigen is unable to initiate immunity, but is able to react specifically with the product of an already established immune response. The HC antigen preparation is neither immunogenic nor tolerogenic alone. However, an antibody response to allogeneic cells can be prevented when these rat HC preparations are chemically coupled to tolerogenic carrier as described in the accompanying paper.