Four hundred twenty patients with open dog bite wounds on initial presentation to the emergency department had Gram stains, aerobic and anaerobic cultures, and antibiotic sensitivities of the bacterial isolates of each wound. Forty-eight percent of the wounds evidenced no bacterial growth. Staphylococcus epidermidis was the most common isolate (20.5%). Multiple bacteria were isolated in 15.5% of patients. Fifty percent of clinically infected wounds showed multiple pathogenic organisms, on both culture and Gram stain. The predominant pathogenic bacteria in clinically infected wounds were, in order, Enterobacteria, Pseudomonas, Staphylococcus aureus, Bacillus subtilis, and beta-Streptococcus. Gram stains were useful in determining specific antibiotic therapy in only 6% of infected cases and in 2% of uninfected cases. Statistical differences appeared between initially clinically infected and uninfected dog bite wounds, in both the types of bacteria and frequency of occurrence. Clinical infections were treated effectively with cephradine in more than 95% of cases. Infected wounds growing multiple pathogenic bacteria and/or Gram stains that showed multiple bacterial types were covered by a cephalosporin or penicillinase-resistant penicillin. Six percent of isolates could require an aminoglycoside for treatment.