The presence of precipitin, complement-fixing and immobilizing antibodies in the serums of rabbits immunized with paramecia is demonstrated. A standard technic for performing the immobilization-test is presented. This test is applied to the investigation of serologic specificity in Paramecium. Paramecium multimicronucleatum may be immobilized by anti-multimicronucleatum serum, but P. aurelia and P. caudatum are not immobilized by anti-multimicronucleatum serum. Analogous specificity is exhibited by anti-caudatum serum and by anti-aurelia serum. When a number of stocks of a single species of paramecium isolated from geographically remote localities are tested with the antiserum prepared with one of these stocks, one or more of the stocks is immobilized; others are not. This is true for each of the three species studied, and, therefore, for each there is evidence suggestive of the existence of two or more antigenic types.