A molecular model of a carbohydrate-protein conjugate is described, involving the non-toxic mutant protein CRM197, serologically related to the diphtheria toxin, covalently bound to a characterized oligosaccharide derived from the molecular structure of type 6A pneumococcal capsular polysaccharide. Physicochemical and immunochemical characteristics of this oligosaccharide-protein conjugate were consistent with a molecule showing a molar carbohydrate/protein ratio of 8, an av. mol. wt of 75,000, and retention of complete immunochemical identity when tested towards the homologous antisera. The immunological characteristics obtained after immunization of 2 animal models showed a high immunogenicity of the glycoconjugate specifically directed towards diphtheria toxin and the type 6A pneumococcal capsular polysaccharide.