The inability of bovine antibody, in contrast to rabbit antibody, to fix guinea pig C′ with soluble antigen preparation derived from M. mycoides was shown to be due to a requirement for a heat labile serum factor. A satisfactory C′-fixing reaction could be obtained using unheated immune bovine serum or heated immune bovine serum to which normal bovine serum, or serum from other species, had been added. The heat labile factor appeared to be neither properdin nor the second, third or fourth components of C′. Similarities between C′-fixing enhancing material and C′1 were noted but no conclusion with respect to their identity is warranted at this time.