The antimicrobial activity of cerium nitrate and silver sulfadiazine was assessed in vitro and in vivo using Pseudomonas aeruginosa. In vitro, the activity of silver sulfadiazine was significantly greater than that of cerium nitrate. Synergism between silver sulfadiazine and cerium nitrate was observed in water or saline solution suspensions, but not in broth. In vivo, cerium nitrate offered no therapeutic benefit in reducing wound infection in contaminated wounds. Treatment of similar wounds with silver sulfadiazine resulted in a significant decrease in wound infection and in the level of viable bacteria when compared with that for untreated controls. The addition of cerium nitrate to silver sulfadiazine in aqueous soultion reduced the therapeutic benefit of silver sulfadiazine.