This study was conducted to evaluate the efficacy and safety of intravenous sulbactam/ampicillin followed by oral sultamicillin. Parenteral sulbactam/ampicillin was administered for 7 to 14 days to 152 in-patients with moderate to severe infections. All patients were treated with sulbactam/ampicillin, but only 140 patients received oral sultamicillin therapy. Eighty-nine men and 63 women participated in this study. Infections included intraabdominal (42 cases), respiratory tract (52 cases), skin and soft tissue (29 cases), urinary tract (16 cases), and miscellaneous infections (14 cases) that included typhoid fever, gastroenteritis, septicemia, and surgical wound infection. Six (4%) patients reported six study drug-related adverse experiences. Gastrointestinal side effects were most common and included epigastric burning and indigestion. Diarrhea was not reported and no patient discontinued drug therapy because of an adverse event. Laboratory abnormalities were infrequent and clinically insignificant. Overall, 98% of the 114 evaluable patients achieved clinical cure or improvement following treatment with sulbactam/ampicillin and sultamicillin. Cured or improved patients in each diagnostic group were 97% for intraabdominal infections, 100% for respiratory tract infections, 100% for skin and soft tissue infections, 100% for urinary tract infection, and 91% for other types of infections. Only 2 (2%) patients were judged to be treatment failures. Microbiologic efficacy, or eradication, was 86% overall, ranging from 75 to 100%. Persistence of pathogens occurred in 5%, and eradication with development of a superinfection occurred in 4%. Fifty-seven percent (30/50) of the isolates tested were resistant to ampicillin alone whereas only 21% (9/42) were resistant to sulbactam/ampicillin (p = 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)