33 renal transplantations in the isohistogeneic rat strains DA/Han and BDE/Han were performed. The transplanted animals were killed at different days after transplantation and the following dates investigated: 1. number of rosette forming spleen cells with donor erythrocytes, calculated for 10,000 spleen cells and to the total number of extracted spleen cells, 2. haemagglutinin titers with donor erythrocytes. 3. titers of serum IgG, 4. Total number of spleen cells, weight of spleens, weight and volumes of kidney transplants and the number of peripheral blood lymphocytes and granulocytes. The results deomonstrate that the method of immunocytoadherence as described by BIOZZI and ZAALBERG in xenogeneic transplantation systems can also be used in allogenic systems. The course of the immune response was biphasic. Quantitation of allorosette forming cells can be considered as a paramter of allograft rejection. The current literature as to the T- or B-cell specificity of rosetting cells is discussed.