Two methods are described for the purification of J chain from polymeric IgA after mild reduction without the use of alkylating or dissociating reagents. The released peptide was separated from other protein components by immunoadsorption combined with gel filtration or anionic-exchange chromatography, or both. J chain was thus obtained in a yield of about 30% of the total release. Most of it consisted of dimers (molecular weight, approximately 25,000 to 30,000) or larger polymers, but re-reduction and alkylation produced a quite homogeneous fraction that sedimented slightly more slowly than egg-white lysozyme. The purity was high enough for successful immunization. When J chain coupled to bovine serum albumin was used as an antigen, all of five rabbits showed a good immune response. Although the same principle could be used for the purification of J chain from IgM and colostral IgA, high purity was more difficult to achieve and the yield was much lower. These preparations contained an unidentified slow-moving component, and the J chain was more prone to become rapidly degraded to smaller fragments.