Secretory IgA (S-IgA), in dissociating media, released small portions of secretory component (SC) and light chains but no J chain. After oxidative sulfitolysis, or reduction and alkylation followed by gel filtration in 5 M guanidine-HCl, SC was eluted in the heavy chain fraction, and J chain in the light chain fraction. A considerable portion of SC and J chain was released by oxidative sulfitolysis in the absence of dissociating agents and separated by gel filtration in neutral buffer. Data indicate one SC and one J chain per molecule of 11S S-IgA. The proposed molecular formula is (α2L2)2J1SC1.