The reactions of species and organ non-specific mitochondrial IgG antibodies were examined by chessboard titrations in an indirect immunofluorescent (IIF) staining system. A conjugate with a molar F:P ratio of 4.3 gave higher titers than a conjugate with a molar F:P ratio of 1.6. A conjugate with a molar F:P ratio of 8.6 gave higher titers than the other conjugates, but this conjugate exhibited a high degree of non-specific staining and was therefore not usable. The range of the plateau endpoint concentration of antibody protein in the conjugates was 3.1 to 12.5 μg/ml. Fixation of the tissue sections gave either lower titers or lower staining intensity than unfixed sections. The antibodies were shown to fix complement in a complement IF method, and the titers were comparable to the titers obtained by IIF titrations.