Native IgM was reduced or reduced and alkylated in the presence or absence of urea, and the preparations were analyzed by electrophoresis in alkaline or alkaline‐urea polyacrylamide gels. All methods easily showed J chain, although the J‐chain pattern varied. Purification of J chain and of IgM depleted of J chain (H.L) was performed by preparative block electrophoresis of reduced IgM Reduction with mercaptoethanol of H.L. and of native IgM followed by reassociation experiments showed that H.L and native IgM had the same abilities to form polymers The reassociated polymers from native IgM sedimented in the ultracentrifuge more slowly than or at the same rate at 19S IgM (called a‐ and b‐component, respectively). After purification of these reassociated polymers, polyacrylamide gel electrophoresis showed that the polymers contained less J chain than the native IgM. Reassociated H.L also contained a‐ and b‐components, but polymers sedi men ting slightly faster than 19S IgM dominated (c‐component). a‐, b‐, and c‐components were also found in a native IgM lacking J chain. Reassociation studies performed on H.L added to the original amount of J chain again showed only a‐ and b‐polymers It was concluded that J chain influences the mode of reassociation but should not be considered a requirement for the polymerization of IgM.