A series of experiments showed that the inoculation of spleen and lymph node cells from rats immunized with Friend lymphoma (WFT-13) CELLS INTO Friend virus-tolerant rats induced the runting syndrome in nearly all cases, and immunological tolerance to WFT-13 was not broken in any survivors. However, th inoculation of specific immune spleen and lymph node cells admixed with normal bone marrow cells suppressed the runting death. In addition, in these animals the primary lymphomas that ordinarily occur about 200 days after neonatal inoculation of Friend virus did not appear. The mixture of immune spleen and lymph node cells and normal spleen and lymph node cells or normal thymus cells was ineffective in preventing the runting death or the incidence of primary lymphoma. Spleen and lymph node cells from normal rats or rats immunized with antigenically different AH-66 cells were also without effect. Spleen and lymph node cells from rats immunized with sheep red blood cells had a relatively high incidence of the runting syndrome; a few survivors rejected the WFT-13 transplants and also did not develop primary lymphomas. These results suggest that a supplement of hematopoietic stem cells from bone marrow eill not only prevent the runting death of Friend virus-tolerant rats produced by inoculating immune lymphoid cells but will also prevent the expected occurrence of primary lymphomas.