This study compared four methods for serotyping isolates of Haemophilus influenzae. Slide agglutination with commercial antisera (Difco Laboratories and Wellcome Diagnostics), coagglutination (Phadebact Haemophilus Test [Pharmacia Fine Chemicals]), latex agglutination with affinity-purified anticapsular antibody, and counterimmunoelectrophoresis with multiple antisera were used to serotype 80 isolates of H. influenzae. Coagglutionation and counterimmunoelectrophoresis correctly identified all 80 isolates as either type b or not type b. Slide agglutination and latex agglutination each successfully identified 76 of the 80 isolates; however, each of these two methods failed to type four isolates because of agglutination of controls. We recommend slide agglutination or coagglutination as the serotyping methods of choice in most laboratories because they are simple, accurate, and rapid. Slide agglutination with Difco antiserum can be performed at the lowest cost.