Horse anti-dog and anti-mouse ALS were digested for varying intervals with acid Taka-protease, a proteolytic enzyme. A progressive reduction in antigenicity was demonstrated, as determined by in vitro precipitin reactions with standard antisera and by passive cutaneous and systemic anaphylaxis tests. Leukoagglutinins and lymphocytotoxins were conserved in anti-dog ALS but there was a marked diminution in anti-platelet and anti-erthrocyte activity. Both anti-red cell and anti-white cell antibodies of anti-mouse ALS were reduced. Digestion appeared to completely degrade some protein fractions such as albumin and to alter the immunoglobulin molecule, possibly by cleavage of the terminal portion of the Fc fragment. Digestion of ALS with Taka-protease may prove to be a useful procedure particularly since it was demonstrated that immunosuppressive potency was partially retained. For further evaluation a better standardized Taka-protease will be required.