A chemotactic factor for rabbit and human polymorphonuclear leukocytes antigenically related to C5 has been isolated from human serum activated by zymosan in the presence of ε-amino caproic acid (EACA). Antiserum to the purified chemotactic factor as well as antiserum to human C5, inhibitis the chemotactic activity in human serum activated by zymosan, aggregated IgG, immune complexes, or bacterial lipopolysaccharide. The chemotactic activity of bacterial (E. coli) factor is not inhibited by either antiserum. The C5 chemotactic factor is purified by a combination of gel filtration and ion exchange gel chromatography. Analysis for anaphylatoxin activity in fractions from each step of the procedure indicates that the muscle-contracting activity is separated from the bulk of the chemotactic activity in the anion exchange step. The purified chemotactic factor migrates as an α globulin as assessed by elution activity in agarose.