Hydrogen ion titrations were done on the undenatured and the heat-denatured (40°, 30 min, pH 6.5) forms of a purified and physically characterized salt-extracted collagen. Exposure to low pH of the undenatured soluble collagen resulted in the unmasking of 7.5 carboxyl groups/105 g. The release of these groups was not associated with detectable loss of helical structure. Either heat or high pH denaturation resulted in the unmasking of 5.0 carboxyl groups/105 g, 1.0 imidazole or a-amino groups/105 g, and 5.5 e-amino groups/105g. The release of these groups was associated with loss of helical structure and separation of the protein to random coils. The unmasking of these ionizable groups is thought to result from the disruption of interchain bonds involving these groups. The precise chemical nature of these bonds cannot be determined from the titration data.