Haemophilus influenzae is an important pathogen in respiratory infections in children and often is implicated in otitis media. It is sensitive in vitro to a number of antibiotics, some of which are used clinically for the treatment of such infections. We have checked the in vitro sensitivity of a type b strain of H. influenzae. When tested in Levinthal’s broth prepared with laked rabbit blood, the culture was most sensitive to tetracycline, ampicillin and penicillin and was somewhat less sensitive to cephalothin, fosfomycin, cephaloridine, and chloramphenicol. However, when this same strain was used to infect mice, fosfomycin was more active than ampicillin, tetracycline, chloramphenicol, penicillin or the cephalosporin