A 19-month study of group B streptococcal infection was performed to investigate the spectrum of such infections in adult males, the relation of serotypes to clinical illnesses, the effects of previous antibiotic therapy on infection and colonization, and the antibiotic susceptibility pattern of these organisms.Twenty-four patients had definite or possible infections while 41 patients were colonized with group B streptococci. The most frequent infections encountered were pneumonia (ten cases) and soft tissue infections (nine cases). Five infections (21%) were nosocomial in origin. The most frequent serotypes were la and II. No correlation of serotype and type of infection was observed. Patients receiving previous antibiotic therapy were significantly more likely to be colonized than infected with group B streptococci. Penicillin was the antibiotic to which these organisms were most susceptible; tetracycline and gentamicin showed the least activity.