Hapten-binding properties of the rabbit IgM antibodies obtained by immunizing rabbits with salmonella cells coupled with 1-azo-naphthalene-4-sulfonate group were determined. It was reconfirmed that the IgM antibodies had 10 detectable combining sites and that half of the total combining sites in the preparation were much higher in affinity for hapten than the other half. The antibodies were separated into two fractions by adsorption with an insoluble immunoadsorbent. The results of hapten-binding experiments with these antibody fractions by equilibrium dialysis showed that each fraction contained the high- and low-affinity sites in about equal numbers, and that equal numbers of high- and low-affinity sites appeared to be present on a single molecule of the antibody. The F(ab′)2 fragment prepared by peptic digestion of the IgM antibody showed similar hapten-binding properties to those of the native antibody, indicating that the observed difference in the affinity of the combining sites was not related to the state of assembly of the subunits in the native molecule. A significant decrease of the combining sites was observed with the peptic monomer fragment, Fab pep, and it seemed that the low-affinity sites were more affected than the high-affinity sites in the process of peptic digestion.