Summary: The ProC′ activity of swine serum was abolished by treatment with dilute formalin. Sera treated in this manner fixed guinea pig C′ in the presence of antigen, but serum titers were somewhat depressed. Because of the prozoning activity of untreated swine sera, little or no indication of fixation occurred with some sera of low antibody content. However, removal of the ProC′ activity of these sera with formalin either abolished or markedly reduced the prozone effect so that a C′F reaction could be obtained.