SummaryThe soluble surface proteins isolated by Cross (1975) fromTrypanosoma bruceiare compared to those which occur onParamecium aurelia. Calculations based on the amino acid residue data and the molecular weights of these surface antigens indicate that there is a considerable degree of similarity between theT. bruceiand theP. aureliaproteins, and that in neither case can simple mutation be responsible for the differences of molecules within each family of antigens. The analyses are consistent with the suggestion that theP. aureliasystem can serve as a model for the phenomenon of phase transformation (relapse syndrome) which occurs in trypanosomiasis.