Studies were undertaken to determine whether broth microdilution susceptibility tests could be standardized by using a single medium for testing fastidious respiratory pathogens. Mueller-Hinton broth with lysed horse blood and the broth version of Haemophilus Test Medium (HTM) were directly compared. Ten orally administered agents were found to give essentially identical results in both media but minor differences were noted. Because the test are easier to read when HTM broth is used, that medium is to be preferred for routine testing of Haemophilus influenzae, Streptococcus pneumoniae, Streptococcus pyogenes and Moraxella catarrhalis isolates by the microdilution procedure.