Sera, purified myeloma proteins, and urinary proteins obtained from eight patients with IgA multiple myeloma were studied by physical-chemical and immunochemical methods. In six patients whose serum viscosity was increased, the sedimentation constants of the principal component of myeloma proteins ranged from 9.1 to 10.2 S. In two patients with nearly normal serum viscosity, the sedimentation constants of these proteins were 6.2 and 7.2 S. IgA-albumin complexes were detected in most of the sera, but in varying amounts; no complexes of Ig with amylase, secretory component, or α1-antitrypsin were observed. Studies on isolated myeloma proteins revealed that all IgA proteins from sera with increased viscosity represented true polymers, linked by disulfide bonds, rather than noncovalently associated aggregates; J chain was detectable by both alkaline-urea disc electrophoresis and immunoelectrophoresis with a monospecific anti-J chain serum. Increased serum viscosity was not related to the IgA subclass, L chain type, or the carbohydrate compositions of individual IgA myeloma proteins. The urine of five patients contained free light chains corresponding in type to the light chain of the particular IgA myeloma protein. However, free J chain was not detected. The immunoelectrophoretic analysis for the presence of J chain in sera of myeloma patients may be used for early and simple detection of polymeric forms of myeloma proteins. © 1977.