The avidity characteristics of salivary and serum antibodies have been determined in 179 salmonellosis patients by means of the indirect hemagglutination inhibition test. These investigation have shown that, in contrast to serum antibodies, no changes occur in the avidity of salivary antibodies in the course of the disease. This is due to the fact that secretory antibodies belong to IgA which have no tendency to ripening. The regularities thus established reflect the role of secretory and serum antibodies in the pathogenesis of Salmonella infections.