J chains purified from colostral IgA, polymeric (mainly dimeric) polyclonal IgA from serum, and monoclonal IgM appeared to be immunochemically identical in tests with antisera produced against the two latter antigens. None of the antisera precipitated with the native immunoglobulins, but J-chain antibodies became bound to all three types of Ig polymer. On a molar basis polymeric IgA was almost twice as efficient as IgM and 17 times more efficient than colostral IgA in antibody inhibition tests. The J-chain antibodies were able to link two or more IgA dimers and two or more IgM pentamers. After incubation in large antibody excess, the sedimentation properties of most IgM molecules were altered, whereas about 40% of the IgA dimers were unaffected by the treatment. THus, although the J chain on the whole was slightly more exposed in polymeric IgA than in IgM, it was more homogeneously available in the latter polymer. After denaturation in acid urea, the J chain became markedly more exposed in all three types of Ig polymer, and they were all precipitated by the most potent antiserum. Immunochemical quantitation of J chain released after reduction with 20mM dithiothreitol indicated that on the average there are about 2 mol of J chain per mol of polymeric IgA, and 3 to 4 mol per mol of IgM.