Eighty rabbits were immunized with native or denatured acid-soluble calf skin collagen. Fifty-five antisera showed haemagglutination titres higher than 1:64. These sera were investigated with calf skin collagen, pepsin-treated calf skin collagen and rabbit skin collagen, used as coating antigens and inhibitors. Three serologically distinct collagen-antibody fractions could be demonstrated: a general, non-species specific fraction reacting with all three antigens, a species specific fraction active with the two calf preparations, and an antibody fraction to pepsin-labile structures of collagen reacting only with untreated calf collagen. The antibody fractions occur in the antisera either alone or as mixtures. The titres of distinct antibody fractions present in the mixed sera were determined by haemagglutination—inhibition experiments.
Antibodies to pepsin-labile structures could be equally well inhibited by collagen peptides obtained after trypsin or collagenase treatment. A ten-fold higher amount of peptides as compared with undergraded antigen was necessary to obtain the same inhibition effect. The results were compared with previously reported peptide inhibition experiments performed with the other two types of antibodies.