(C57BL/6 x A/J)F1 murine recipients of DBA/2 kidney allografts developed tolerance to DBA/2 tissues, which was measured by observation of growth of a DBA/2 tumor. Eleven, 14 and 18 days after inoculation, the size of the tumor was considerably larger in kidney-grafted than in nongrafted animals. Still, the susceptibility of grafted animals to the tumor did not equal the susceptibility of the DBA/2 mice syngeneic with the tumor. Removal of the renal graft left the recipients with significant tolerance which, however, was weaker than that of the mice in which the kidney graft was left undisturbed. In parabiosis experiments, it was noted that the size of the DBA/2 tumor was equal in the partner which received the DBA/2 kidney graft and in the partner which was not grafted. These experiments rather clearly showed that the tolerance studied was of an 'infectious' type and that it was apparently transferred by some suppressing factor(s) present in the circulation.