SUMMARYUsing a monoclonal antibody to H‐Y antigen we characterized the molecular determinant responsible for H‐Y reactivity. H‐Y‐positive cells were treated with enzymes which alter carbohydrate structure and then were tested for their ability to absorb monoclonal anti‐H‐Y antibody. Indications are that the serological determinant recognized by this antibody is a glycoconjugate containing terminal non‐reducing and internal galact