The importance of graft-vs.-host (GVH) activity to the ability of thymocytes to augment hemopoiesis in radiation chimeras was investigated. Parental (P) lymph node cells were found by the 59Fe-uptake method not to have an analogous augmentative effect. When thymus donor, marrow donor, and irradiated recipient were chosen immunogenetically so that GVH could occur in either the presence or absence of graft-vs.-graft (GVG) activity, it was seen that GVH reactivity per se resulted in no improvement of marrow growth. However, when P thymocytes specifically tolerant to an F1 hybrid host were administered with P marrow, augmentation was three times greater than when nontolerant P thymocytes were given. It was concluded that GVH activity not only is not essential but actually is detrimental to augmentation. Ninety-day survival of chimeras given specifically tolerant P thymocytes was better than that of mice given P marrow only and very much better than those given marrow and nontolerant thymocytes.