The inherited complement deficiency of certain inbred strains of mice was shown to be due to an isolated lack of the fifth component of complement. The protein MuB1 (or hc'), which is present in normal mouse serum but absent from the serum of complement-deficient mice, was shown to be immunochemically related to the fifth component of human complement (C'5). C'5 hemolytic activity was specifically inhibited in human serum by mouse anti-MuB1 and in normal mouse serum by mouse antiserum to human C'5. Highly purified human C'5 reconstituted the hemolytic activity of complement-deficient mouse serum. It was, therefore, concluded that he', or MuB1, constitutes the murine analogue of the fifth component of human complement.

The MuB1 concentration in normal mouse serum was found to be subject to a sex-related variation. By quantitative precipitin analysis it was demonstrated that serum from male mice contains twice as much MuB1 as that of female mice. This difference in C'5 concentration was also detected by hemolytic assay. In addition, C'6 and C'7 were also found to be subject to sex-related variations.