Protein Fv, a human sialoprotein recently described in the stools of patients suffering from liver diseases, binds the variable domain of H chains without impairing Ag binding. Normal subjects are shown here to secrete protein Fv under a hidden form, saturated with luminal Ig. In feces, the nonimmune complexes are essentially of 1800 and 800 kDa M(r); they contain protein Fv molecules bound with F(ab')2 fragments produced by cleavage of Secretory IgA during colonic transit. The 800-kDa complexes correspond to 6 molecules of F(ab')2 fragments bound to a sole protein Fv dimer. This was established by comparison with an in vitro-made complex having a valency of 6. Investigation of the role of protein Fv shows that in vitro addition of this free molecule to antivirus or to anti-Salmonella typhi antibodies allows or augments agglutination of the corresponding pathogens. This property is of major interest for secretory antibodies because it favors their role in Ag conveyance in the mucus stream. It seems therefore that protein Fv is a novel key factor of the immune defense in gut.