We describe the use of a DNA probe for genotyping clinical isolates of Haemophilus influenzae. The probe, containing capsulation genes, differentiates between the six Haemophilus serotypes in a Southern blotting procedure. It also hybridizes with a distinctive pattern to DNA from capsule-deficient mutants of serotype b strains, while failing to hybridize to DNA from typical clinical isolates of non-serotypable H. influenzae. The probe can thus resolve issues of serotyping uncertainty such as arise, for example, when capsulate strains are found to have lost reactivity with serotyping reagents after storage or transmission from one laboratory to another. The probe has proved useful in the evaluation of Haemophilus infections in infants following administration of H. influenzae type b vaccine. In an illustrative example, the probe was used to resolve serotyping ambiguity in a case of Haemophilus bacteraemia in a vaccine recipient, providing compelling evidence that the organism responsible was neither type b nor derived from a type b strain. The widespread introduction of vaccines against H. influenzae type b disease will increase the importance of the precise identification of strains infecting immunized children. This need can only be met by the development of 'gold standards' such as capsulation gene probes.