EXTRACTS of embryonic chick muscle contain a carbohydrate-binding protein assayed as an agglutinin of treated erythrocytes1–4. Because agglutination is blocked by specific saccharides, the agglutinin is referred to as a lectin by analogy with similar proteins from other sources5. Lectin activity changes strikingly with muscle differentiation3,4 and is detectable on the surface of muscle cells6, suggesting a role in developmentally regulated cell interactions. The lectin has been purified to homogeneity by affinity chromatography6. Lactose is a potent inhibitor of this lectin (50% inhibition at 0.3 mM) whereas N-acetyl-D-galactosamine is not (no inhibition at 150 mM). We now report a second distinct lectin activity discovered during studies with developing chick muscle. This activity is not detectable with the test erythrocytes used in previous studies, but is readily detectable if the erythrocytes are kept in the refrigerator for 2–3 months. The lectin activity identified by these altered erythrocytes is inhibited by N-acetyl-D-galactosamine but not by lactose. Like the lactose-sensitive lectin activity, it changes with muscle development.