Antibodies are described, which are suitable for identification of .gamma.-carboxyglutamic acid (Gla), said antibodies having an ability of identifying Gla residues in proteins and/or peptides, while not reacting with glutamic (Glu) residues in corresponding proteins and/or peptides. Methods for the preparation and identification of said antibodies as well as methods for detection of Gla in biological fluids, tissue extracts, tissue specimens, in which methods said antibodies are used, are also described. There is also described the use of said antibodies for immunopurification of Gla-containing proteins or peptides by, for instance, immunoprecipitation or immunoaffinity chromatography.