An antigen, MuB1, present in the sera of some mice, can elicit a precipitating antibody in certain other strains of mice. An antibody to the antigen MuB1 can also be elicited in rabbits. 99 strains and substrains of inbred mice were tested for the presence of MuB1; the antigen was found in the sera of 44 strains (61 per cent) and 14 DBA substrains (52 per cent). Evidence is presented indicating that mice lacking MuB1 do not make a modified antigen, corresponding to MuB1, but are genetically deficient in synthetic ability at this site. By reaction with antibody to MuB1 an antigen corresponding to MuB1 was found in 13 of the 15 orders of mammals, and in 63 of 85 mammalian species tested, including man and guinea pig. The quantity of the antigen MuB1 is always greater in the serum of male than in the serum of female mice. The concentration of MuB1 increases with age; this increase is more marked in male than in female mice. By means of backcross experiments it was shown that the inheritance of MuB1 is unifactorial, is independent of the inheritance of the gamma globulin allotype MuA2, and is qualitatively independent of the sex of the parents. The antigen MuB1 is found in the euglobulin fraction of serum; it loses its ability to precipitate with antibody after heating at 56°C, but not after treatment with ammonia or hydrazine. By gel filtration, MuB1 is separated with a fraction containing molecules of molecular weight ≈ 150,000. An empirical correlation was observed between the presence or absence of MuB1 in the sera from inbred mice and the presence or absence of hemolytic complement (Hc), as measured by a test using a high concentration of rabbit hemolysin. In backcross experiments also, a correlation between hemolytic complement and the presence of MuB1 was demonstrated. As with MuB1, male mice had a higher hemolytic complement level than females. The particular component of complement which may be identical with MuB1 has not been identified.

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