Ninety commercially supplied sera for the capsular typing of Klebsiella species were tested in a full chequerboard against 72 type strains. Only four of 144 homologous reactions could not be detected. Of the 6336 possible heterologous reactions, 91 actual cross reactions were found. All positive reactions were titrated and working dilutions deduced. This provides essential information for the successful typing of unknown strains with these sera.