A 51Cr release microhemolytic complement assay is described to detect hemolytic complement activity in mouse serum, 51Cr-labeled sheep erythrocytes (SRBC) which have been sensitized with a subagglutinating amount of the 7S IgG fraction of rabbit anti-SRBC serum are placed in microtiter plates and further antibody added to each well prior to the addition of the complement (C) source. The IgG antibody was found to be more efficient in the lytic assay than the 19S IgM antibody. The assay is simple to perform, reproducible, and requires small volumes of mouse serum. Comparative hemolytic values were established for serum from a variety of murine strains using pooled BALB/c serum as the C reference. No apparent relationship was noted between the H-2 type of inbred mouse strains and the presence or absence of hemolytic complement.