SUMMARYThe capacity of peripheral venous lymphocytes to synthesize the three major immunoglobulins was tested in chronic lymphatic leukæmia, lymphoma and lymphosarcoma, Hodgkin's disease, reticulum cell sarcoma and infectious mononucleosis.Autoradiographs of immunoelectrophoresis patterns of radioactive proteins synthesized by the lymphocytes were indicative of reduced synthesis of all types of immunoglobulins in most cases of these groups. No pattern of deficiency of synthesis was constantly related to any disease.Normal synthesis of immunoglobulins was seen in one of three cases of Hodgkin's disease and one of 10 cases of lymphosarcoma. In two cases of lymphosarcoma labelling of IgA was much heavier than labelling of IgM and IgG. This was thought to indicate a specific derangement of immunological mechanisms, rather than a general impairment of immunological activity.Very heavy synthesis of all three immunoglobulins was found in one of II cases of infectious mononucleosis. No clear pattern of activity and quiescence was seen when these cases were arranged in order of the length of the illness.