Summary:Cellular immunity has been studied by means of lymphocyte response to PHA, delayed hypersensitivity and skin window responses in 23 patients with myeloma (14 1gG, 9 1gA) and 14 patients with macroglobulinaemia.In the myeloma patients, 14% had abnormal PHA response and 29% were anergic. In those with macroglobulinaemia, 29% showed abnormal PHA response and 57% were anergic. In myeloma, the abnormal PHA response was due to a serum inhibitor.Abnormal skin window responses were present in 75% of the patients with myeloma, but only 22% of those with macroglobulinaemia. All the myeloma patients with anergy had abnormal skin windows but this correlation did not exist in macroglobulinaemia.No correlation was found between the paraprotein concentration and anergy, PHA response or skin window.The results support the conclusion that myeloma is predominantly associated with an abnormal skin window (inflammatory) response and macroglobulinaemia with intrinsic abnormalities of cellular immunity. When anergy and abnormal PHA response are present in myeloma, it appears to be attributable to an effect of the paraprotein and not an intrinsic abnormality of lymphocytes.