Grafting of the spleen as an organ of full immunological competence in inbred rats was found to be a useful tool for studies of immunological parameters, which would be difficult to evaluate in outbred and larger animals. Early results indicate that the spleen is under certain circumstances less susceptible to rejection than other organs in the rat and knowledge about principles, underlying this phenomenon, might have application for grafting of organs in other species. Experimences gained in grafting more than 400 rat spleens heterotopically were described together with the technique employed.