Summary: A test has been devised to measure hemolytic complement activity of mouse serum. The chief requirement for detecting mouse hemolytic complement is the use of extremely large amounts of sensitizing antibody. The hemolytic factor measured in mouse serum is indeed complement. This is indicated by the thermolability, sensitivity to EDTA, and failure to lyse unsensitized cells of the hemolytic factor. Among several strains of inbred mice tested, two were found to be lacking hemolytic complement. One of these strains, B10·D2 is supposedly co-isogenic, except for the histocompatibility locus H-2, with the strain C57BL/10SnHz which contains complement. The data suggest that the two strains must differ by at least one additional gene, and this one is a determinent of complement activity.