A lectin which binds specifically to beta-D-galactoside groups is present at the earliest stages of chick embryo development.

Extracts obtained from chick embryos at the pre-gastrula and gastrula stages are able to agglutinate trypsinized rabbit erythrocytes fixed with glutaraldehyde. Agglutination is inhibited by saccharides sharing a beta-D-galactopyranoside configuration. Agglutinin activity is also inhibited by desialysed fetuin which bears terminal galactose residues but not by native fetuin, desialysed agalacto-fetuin, bovine submaxillary mucin and desialysed bovine submaxillary mucin. Lectin activity is present in extracts obtained from the embryonic area of the blastoderms as well as in extracts from extra-embryonic endoderm and ectoderm. In extracts subjected to gel filtration on ECD Sepharose, lectin activity eluted between ovalbumim (mol. wt 45 000) and chymotrypsinogen (mol. wt 25 000). Under some experimental conditions, thiodigalactoside, the most potent inhibitor of lectin activity, inhibited the aggregation of cells of the extra-embryonic endoderm of the primitive chick embryo.