Lectins that agglutinate intact red blood cells of rabbit, and their endogenous inhibitors, were found in extracts of cow, pig, rat, and human kidneys. Lectins were separated from inhibitors by gel chromatography on a column of Toyopearl HW-75. Adsorption tests with immobilized glycoconjugates and ion-exchange gels revealed that the bovine kidney lectin binds with sialylglycoproteins by sugar-specific interactions but not by nonspecific, electrostatic interactions. The results of the hemagglutination-inhibition tests showed that all of the endogenous inhibitor fractions were active toward all renal lectins tested, and that glycoproteins having sialyl residues, and also heparin, are also good inhibitors of the lectins. However, the hemagglutination-inhibitory activity of human urinary Tamm-Horsfall glycoproteins varied from source to source, and that having the highest inhibitory activity did not always have the highest content of sialic acids. Crude bovine kidney lectin was further purified by ion-exchange chromatography on DEAE-Sephadex A-50. Analysis by SDS-polyacrylamide gel electrophoresis showed that the purified lectin consists of a approximately 63,000 daltons subunit.