The properties of a previously purified beta-galactoside-binding lectin of human placenta were studied in detail. Isoelectric focusing gave multiple bands around pH 4.9, although the lectin preparation was homogenous in SDS-polyacrylamide gel electrophoresis. High-performance gel chromatography suggested that the lectin exists mainly as the monomer and that a small fraction forms a dimer. From all the criteria examined, human placenta lectin resembles one of the chick lectins obtained from embryonic skin or adult intestine (subunit molecular weight: 14,000). The lectin was inactivated by thiol-modifying reagents, p-chloromercuribenzoic acid and N-ethylmaleimide. Reduced and carboxymethylated lectin contained five carboxymethylated cysteines per subunit, and five free thiol groups were titrated by using 5,5'-dithio-bis(2-nitrobenzoic acid). Preliminary sequence analysis showed the presence of a region highly homologous to the corresponding region of the chick lectin (13 identical residues out of 18 from number 70 to 87 of the chick lectin), suggesting a close evolutionary relation between these lectins and the importance of this conserved region in the function of the lectins.