A case of paraproteinaemia associated with pernicious anaemia is described. The patient's serum contained 2M-components (IgM and IgG), while the urine contained gamma-globulin of the L-chain type. All abnormal components belonged to type Kappa. Chromosome studies on blood cultures revealed a mosaic pattern, about one third of the cells having a karyotype identical to that found in Down's syndrome (trisomy of a small acrocentric chromosome), the remainder having a normal male constitution.