A previously developed algorithmic method for identifying a geometric invariant of protein structures, termed geometrical core, is extended to the and 1 domains of immunoglobulin molecules. The method uses the matrix of distances and does not require the usual superposition of structures. The result of applying the algorithm to 53 Immunoglobulin structures led to the identi cation of two geometrical core sets of atom positions for the and 1 domains. Key words: atom coordinate prediction, immunoglobulin geometry, preferred coordinate system, protein core.