Hydrogen ion titrations were used as a method to study the ionizable groups which may be involved in bonding in an apparently highly cross-linked form of soluble collagen. Titrations were done on the undenatured and the heat-denatured (40°, 30 min, pH 6.5) forms of a purified and physically characterized collagen which was extracted from human dermis with pH 1.5 citrate solution. Heat denaturation resulted in the unmasking of 7.5 carboxyl groups/105 g, 1.0 imidazole or α-amino group/105 g, and 6.5 ε-amino groups/105 g. The release of these groups was associated with loss of helical structure and separation of the protein into α and β components (α:β=1:9). The unmasking of these ionizable groups is thought to result from the disruption of interchain bonds involving these groups. Furthermore, after the heat denaturation, not all groups known to be present from amino acid analyses were titrated, indicating the presence of groups which remained masked. The quantitative values for these masked groups were: 5.0/105 g of carboxyl groups, 0-0.5/105 g of imidazole or α-amino groups, and 6/105 g of ε-amino groups. These masked groups are thought to be involved in interchain bonds between the polypeptide chains which make up the cross-linked components.