A prospective, randomized, single-blind comparison of parenteral cefamandole and ampicillin was conducted in 27 hospitalized adult patients with pneumonia or purulent tracheobronchitis due to Haemophilus spp. Patients received either parenteral cefamandole or ampicillin in a dose of 1 g every 6h. Cefamandole was as effective and safe as ampicillin. Of the 14 patients treated with cefamandole, 13 were considered cured, as were 12 of the 13 treated with ampicillin. One patient in each treatment group improved clinically but did not clear his sputum of Haemophilus spp. One patient treated with cefamandole had a recurrence of haemophilus spp. bronchitis 9 days after cure. Adverse effects were more common in the cefamandole-treated group (50% versus 15%), but were mild and did not require discontinuation of therapy in any patient. The in vitro susceptibilities of 64 clinical isolates of Haemophilus spp. to 10 antibiotics were determined. Cefamandole was the most active of the cephalosporin-cephamycin antibiotics teste, inhibiting 98% of 61 non-beta-lactamase-producing isolates at 2 μg/ml and 100% at 4 μg/ml. Cefamandole inhibited the three ampicillin-resistant isolates at 2 μg/ml of less. Cephapirin, cefoxitin, and cephalothin were the next most active, whereas, cefazolin and cephradine were the least active.