Precipitating antibodies to acid-soluble calf-skin collagen which quantitatively precipitate collagen from solution recognize antigenic determinants which are separable from the major collagen subunits by heat denaturation and ion exchange chromatography. This smaller portion of collagen which retains full antigenicity is collagenous in that immunologic activity is completely lost by collagenase digestion and in its content of hydroxyproline and glycine, the latter in just less than one residue out of three. Some molecular size and charge characteristics of the antigenic moiety are described.