Complement-fixing properties of pepsin-treated rabbit and sheep antibodies.

Rabbit antibody with a sedimentation coefficient of 7S was digested with pepsin to yield the 5S fragment. Washed specific precipitates of 5S antibody and egg albumin, when added to guinea pig serum, fixed up to 40 percent of the total complement. This fixation could not be attributed to contamination with 7S antibody. Absorption of guinea pig serum with washed precipitates of 5S antibody and antigen removed that portion of complement which was fixed by fresh 5S antibody-antigen aggregates, but not that fixed by 7S antibody-antigen aggregates. The presence of 0.001M ethylenediaminetetraacetic acid prevented this absorption of complement by the 5S aggregates.