Two inbred strains of rat (Donryu and Sprague-Dawley strains) were developed. The skin reactions of these strains immunized with M. tuberculosis, hen egg albumin (OVA) or hen egg lysozyme and challenged with the purified protein derivative (PPD) or each antigen were even and uniform. The Donryu strain showed a typical Arthus reaction with petechiae and edema and a negligible delayed skin reaction, whereas the Sprague-Dawley strain showed a poor Arthus reaction and a typical delayed skin reaction with central necrosis and induration. The Arthus reaction or delayed skin reaction could be passively transferred to recipient rats of each strain by immune sera or sensitized peritoneal exudate cells (PEC), respectively.