A method for the immunoblot analysis of the type-specific capsular polysaccharides of streptococci was developed. The capsular polysaccharides were extracted by sonication and subjected to polyacrylamide gel electrophoresis in the absence of sodium dodecyl sulphate (SDS). After electrophoresis the polysaccharides were transferred to charged PVDF-N membranes and probed with the type specific antibodies. A characteristic ladder-like pattern of polysaccharide bands specific for the serotype (1, 2, 4, 7) was observed for capsular extracts of Streptococcus suis. Human immune sera against type-specific group B streptococcal polysaccharides reacted most strongly with the immunizing polysaccharide type (Ia, II, III). The previously observed crossreactions between the group B streptococcal type-specific capsular preparations were shown to be due to binding to the isomeric polysaccharide molecules. Thus, gel electrophoresis combined with immunoblot analysis seems to provide a novel method for the molecular and immunochemical characterization of bacterial polysaccharides and for the study of the specificity and properties of antibodies to capsular polysaccharides.