Pepsin digestion of precipitating 7S rabbit antibodies yields a smaller 5S
fragment which is still capable of precipitating homologous antigen (1). This
digestion is thought to remove most, if not all, of Porter fragment III (1).
Treating the 5S fragment with cysteine yields two fragments, which behave as
Porter fragments I and II (1). Fragment III, when aggregated, can fix complement;
fragments I and II cannot (2). The conclusion has been drawn that the
complement-fixing ability of antibody is due to fragment III, and that the 5S
fragment is, therefore, incapable of fixing complement (3).
Reinvestigation of this problem originally for other purposes, led to the
finding that 5S rabbit, as well as 5S sheep antibody does retain certain complement-
fixing abilities. However, as we intend to show, the complement-fixing
abilities of 7S and 5S antibodies are both quantitatively and qualitatively
different. A preliminary report of some of these findings has appeared (4).