Secretory component (SC), synthesized as a transmembrane protein, acts as the receptor that binds IgA dimers and mediates their transepithelial transport. Cleavage of the receptor (membrane SC) apparently occurs during transport and a fragment, the secreted form, is generated, which remains tightly bound to the IgA dimer. In the rabbit, variation in the size of membrane SC is observed with both a high and a low molecular weight family, each composed either of two or of four distinct polypeptides depending on the individual rabbit. The same degree of size heterogeneity is observed for secreted SC. Part of this size heterogeneity is related to genetic polymorphism. The milk of individual rabbits typed with anti-SC-allotype sera reveals three different banding patterns. The simplest pattern, found in t61/t61 and t62/t62 homozygotes, consists of an upper and a lower doublet. Since each band of these two doublets in the t62 allotype migrates slightly faster than its counterpart in the t61 allotype, a composite pattern is observed in the heterozygotes (t61/t62). Within a given allotypic group, all SC polypeptide chains expressed the identical allotypic specificity. The 2000 to 4000 difference in molecular weight between the two forms of a doublet probably reflects differences in the number of glycosylated asparagine residues, since individual bands of a doublet show identical peptide maps. High and low molecular weight families are also structurally related to each other as shown by one-dimensional peptide maps and identical NH2- and COOH-terminal amino acid sequences. These results indicate that the 25-kDa size difference between SC from the high and low molecular weight families reflects an intramolecular deletion.