A polypeptide chain, other than the heavy and light chains and the secretory component, has been observed in secretory immunoglobulin A (IgA) of both rabbit (1) and human (2) origin. This J chain, as it is called, is readily detectable as a fast-moving band on alkaline-urea disc electrophoresis gels of L-chain fractions from reduced and alkylated or S-sulfonated polymeric immunoglobulins (1, 2). It has been suggested that this component functions to join together the two 7S subunits of both 11S secretory and serum IgA (1). The presence of an analogous chain was recently described in studies involving human macroglobulinemic IgM (2) and in ovine (3) and porcine (Zikan, personal communication) secretory immunoglobulins. In the present investigation several other animal species representing key positions on the phylogenetic scale were examined in an attempt to establish whether or not this J chain is a common component of phylogenetically distinct polymeric immunoglobulin molecules.