Mouse Bf measured by indirect lysis with glutathione-treated cells was shown to be present in high quantities in chemically induced mouse ascites fluid (MAF). Functional MAF Bf has physical properties identical to mouse serum Bf. It is heat labile, Mg++ dependent, and is activated by purified CVF. Purification was carried out with Bio-Rex 70, CM 32, and DE 52 column chromatography. The optimum isoelectric pH was previously determined by isoelectric focusing. Fractions for Bf were tested by immunochemical means by using cross-reacting goat antihuman Factor B and by functional assays. Antiserum to purified Bf was prepared in the goat. By immunoelectrophoresis and rocket electrophoresis MAF Bf and serum Bf migrated anodally. Cleavage of a cathodal migrating Bf fragment was observed with purified CVF. Bf phenotypes were typed by the immunofixation procedure. Sera from male and female mice of 16 inbred strains were examined (A/J, A.KR, CBA, C3H, B10.BR, C58, B10, B10.A, C57L, C57 BL/Ks, B10.D2, DBA/2, DBA/1, BALB/C, 129/J).