A technique for purifying rat monoclonal antibodies from ascitic fluid or serum is described which is based on 2 facts. First, approximately 95% of rat immunoglobulin light chains are of the kappa type. Second, an allotypy in the rat species is located on the constant part of the kappa light chain. By use of a mouse monoclonal antibody with specific binding affinity for the Ig kappa-1a allotype on the kappa light chains of the LOU inbred rat strain, it is possible with immunoaffinity chromatography to isolate LOU Ig kappa-1a-bearing immunoglobulins from the serum proteins, including the immunoglobulins, of rats of Ig kappa-1b allotype. LOU histocompatible hybridomas synthesizing the Ig kappa-1a allotype can be transplanted into rats congenic with the LOU inbred strain carrying the Ig kappa-1b allotype, since LOU rats with the Ig kappa-1a kappa light chain allotype and congenic LOU Ig kappa-1b rats with the Ig kappa-1b kappa light chain allotype are fully histocompatible. The serum or ascitic fluid of the recipients is applied to an immunoabsorbent column to which mouse monoclonal antibody against the Ig kappa-1a allotype is coupled. The serum proteins, including the host immunoglobulins pass through the column. An appropriate buffer is used to elute the monoclonal antibodies in a second step. The same technique may be employed for other monoclonal antibodies. A reciprocal system using mouse monoclonal antibodies against Ig kappa-1b rat allotype can be used, a plasmacytoma or hybridoma synthesizing Ig kappa-1b kappa light chain being transplanted into an Ig kappa-1a kappa light chain synthesizing rat. The method is rapid, efficient and inexpensive. Its limitation is with respect to lambda-type monoclonal antibodies, which are relatively rare.