Lipopolysaccharide (LPS) was extracted from 50 isolates of Haemophilus influenzae by a rapid micromethod and was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining. LPS isolated by this method was electrophoretically indistinguishable from conventionally purified LPS but lacked the large degree of heterogeneity indicative of variable polymerization of O-side-chain repeating units; one to four bands appeared in gels. Electrophoretic profiles of LPS from H influenzae type b varied among strains and were useful for subtyping. Thirty-three isolates of type b were classified into 11 subtypes on the basis of stable band mobilities. Certain isolates of identical outer-membrane-protein subtype differed in LPS subtype. The LPS profiles of non-b, typable strains of H influenzae were generally similar in microheterogeneity to those of type b strains, while those of untypable isolates usually displayed less electrophoretic variation.