J chain was isolated from a human immunoglobulin A myeloma protein and subjected to sedimentation equilibrium ultracentrifugal analysis in 5 M guanidine hydrochloride. The preparation demonstrated minimal heterogeneity. Apparent molecular weight values for the J chain were not concentration dependent over the range examined. The weight-average molecular weight at zero concentration was found to be 15 600 ± 200. © 1973.