The in vitro and in vivo activities of PC 904 were tested against Pseudomonas aeruginosa isolated from infected eyes and were compared with carbenicillin and gentamicin. By the microtiter plate method, we tested 30 strains of P. aeruginosa in vitro; PC 904 was very active against 29, but one strain was resistant. In a comparative study, PC 904 was more active than carbenicillin but less active than gentamicin. In vivo tests were conducted on rabbit corneas infected with a strain of P aeruginosa isolated in El Salvador. At moderate and high dosage levels, PC 904'S efficacy was comparable to carbenicillin's but much less than gentamicin's. When tested for ocular toxicity, the subconjunctival administration of PC 904 was well tolerated. The drug's greater activity in vitro than in vivo was probably due to its high serum-binding capacity.