A 28-year-old female in Denver was found in early 1974 to have frontal sinusitis, osteomyelitis, and bacteremia due to Haemophilus influenzae, type B. The minimal inhibitory concentration of ampicillin for this organism was 100 μg/ml and the minimal bactericidal concentration was >100 μg/ml. It was inhibited by chloramphenicol at 0.4 μg/ml. Further studies demonstrated that ampicillin and methicillin were synergistic against this organism. It was shown to produce a diffusible beta-lactamase. Transferase of resistance from this organism to a susceptible Haemophilus parainfluenzae and a reciprocal transfer were accomplished. A test for transformation was negative as was a test for reversal of resistance by ethylenediaminetetraacetic acid.