A similar enhancement in normal rats has been obtained with C' component reagents possessing properties associated with the third component of C'.

Parallelisms between in vitro fixation of C' and PCA induction by antigen and antibody are shown. The horse anti-pneumococcus system has low C'-fixing potencies and is also less efficient than the rabbit polysaccharide system in the induction of PCA.

Findings of a similar nature were observed in the reaction of rabbit anti-ribonuclease with ribonuclease, the acetylated and guanidinated derivatives of the enzyme.

The injection of hemolytically active serum into C'-deficient rats was accompanied by a partial restoration of PCA. Restorative effects were also noted with heated and ammonia-treated serum.

The return of hemolytic potency and responsiveness to PCA in C'-depleted rats, follow a similar time course.

The data presented indicate that the PCA reaction can be studied as a function of at least three variables, antigen, antibody, and a serum constituent resembling C'.