1. When two chemically different carbohydrate derivatives are bound to the same protein, the newly formed antigens exhibit distinct immunological specificity. 2. When the same carbohydrate radical is conjugated with two chemically different and serologically distinct proteins both of the sugar-proteins thus formed acquire a common serological specificity. 3. The newly acquired specificity of the artificially prepared sugar-proteins is determined by the chemical constitution of the carbohydrate radical attached to the protein molecules. Simple differences in the molecular configuration of the two isomers,-glucose and galactose-suffice to orientate protein specificity when the corresponding glucosides of the two sugars are coupled to the same protein. 4. The unconjugated glucosides, although themselves not precipitable in immune serum, inhibit the reaction between the homologous sugar-protein and its specific antibody. The inhibition test is specific. 5. The sugar derivatives unattached to protein exhibit the properties of carbohydrate haptens; they are non-antigenic but specifically reactive, as shown by inhibition tests, with antibodies induced by proteins containing the homologous diazotized glucoside. 6. The specificity of artificially prepared sugar-proteins is discussed with reference to the chemo-immunological nature of the bacterial antigens containing complex sugars.