Seitz filtration of Trudeau old tuberculin (TOT) removes the erythrocyte-sensitizing substance without altering the capacity of the TOT to react in'' gel precipitin tests or to inhibit hemagglutination or hemolytic tests. The adsorptive capacity of the Seitz filter is limited. Millipore (HA) and fritted glass (UF) filters had only a slight effect on the erythrocyte-sensitizing-substance content of small volumes of TOT 1:15.