Specific pathogen free rats bearing primary sarcomata induced by subcutaneous implantation of a pellet of 3,4-benzpyrene were studied. No evidence could be found that the pellet of 3,4-benzpyrene exerted any immunosuppressive action even with weak antigens. Yet there appeared to be an immunological defect in these animals since the number of immunoblasts (i.e. large basophilic lymphocytes) in the thoracic duct lymph was not raised above the normal in spite of the fact that these tumours contained specific antigens which on transplantation evoke a typical immunoblast response in syngeneic animals. Within 24 h of surgical excision of these primary tumours, the number of the immunoblasts in the lymph increases sharply. The histological appearance of the nodes draining such tumours was consistent with a vigorous immunological reaction. Characteristically, such nodes contained many plasma cells, which disappeared very quickly after surgical removal of the tumour. The tumour-draining nodes respond extremely well in terms of release of immunoblasts, antibody formation and graft rejection to antigens other than those present in the tumour. The immunological abnormality in these nodes would appear to be specific for the antigen coming from the tumour and it is suggested that continual bombardment of a node by antigen interferes with the normal release of immunoblasts into the efferent lymph and thus prevents their appearance in the thoracic duct lymph. This must affect seriously the reaction of the host against the tumour since this reaction is probably mediated by the immunoblasts. These observations may explain why an autograft from the primary tumour is not rejected so long as part of the tumour remains in situ but fails to grow once the primary tumour has been totally excised. The failure of nodes to release immunoblasts in response to the tumour antigen is limited to the node draining the tumour, since implantation of an autograft to a distant site causes the appearance of immunoblasts in the lymph. These observations may explain why immunization with irradiated tumour autograft sometimes exerts a growth inhibitory effect on a primary tumour.