Tolerance to CBA/Cum skin was induced in C3H/Anf Cum mice by neonatal injection of CBA/Cum spleen cells. The tolerant status of these mice was a specific one, for they were able to produce a normal amount of agglutinins against a challenge dose of rat RBC. The immune system responsible for the specificity of tolerance in these mice was identified to be of host type. No immune activity of donor cells against rat RBC could be detected, whereas donor type antigenicity was found in the spleen of the tolerant mice. On the basis of these findings it was proposed that the definition of tolerance given for the whole animal could also be applied to its immune system.