A 100-kDa protein was isolated from the complement-activating component of mouse Ra-reactive factor. It generated a 29-kDa chain upon reduction. Partial amino acid sequences of the 29-kDa chain were determined after fragmentation with CNBr. A 386-bp-long probe was synthesized by the polymerase chain reaction with oligonucleotide primers designed by reference to the amino acid sequence. Using this probe, we cloned a 3.7-kb DNA from a mouse liver cDNA library. The amino acid sequence of the 29-kDa chain deduced from the nucleotide sequence of this cDNA was consistent with the amino acid sequences determined. The full sequence was found to have characteristics of a serine protease and exhibited 29% and 30% homologies with those of light chains of the C1r and C1s subcomponents of complement, respectively.