Ten patients with chronic lymphatic leukaemia (CLL) were injected with the antigen phichi174 intravenously on two occasions to test their capacity for antibody production. Nine out of ten patients failed to develop a primary response, and five out of ten failed to develop a secondary response. Only one out of the ten of the secondary responses fell within the normal range. There was evidence of defective switching from IgM to IgG antibody in two out of three cases in which the immunoglobulin class of antibody in the secondary response was determined. The severity of the immunoparesis in CLL is related to the severity of the disease.