Immunoblot electrophoresis of serum from a myeloma patient shows a clear abnormality even in the dye-stained first lane - the gamma-globulin band is abnormally narrow and intense. The results in lanes 2 and 6 indicate that this patient bears a tumor which is secreting a large amount of a monoclonal IgG with a lambda light chain. (The appearance of the IgA and kappa-staining in this gel is normal.)