These experiments were undertaken to study the nature of the fragments produced by pepsin digestion of seric IgA. We describe a new fragment with a MW of 140,000 daltons. This fragment, the first produced during pepsin hydrolysis of monomeric IgA, corresponds to an IgA molecule which has lost its CH3 domain. We propose to call this fragment F(abc)'2alpha. It is gradually digested and transformed into an F(ab)'2alpha fragment. When polymeric IgA are digested, an extra fragment with a MW of 290,000 daltons is produced, which has kept its polymeric structure. It corresponds to a covalent [F(abc)'2]2alpha fragment. These findings suggest that there is an intersubunit disulfide bridge in the Calpha2 domain. These results also indicate that polymeric IgA proteins are more resistant to pepsin digestion than monomeric IgA proteins.