The efficacy of cefsulodin against infections caused by Pseudomonas aeruginosa was investigated first in an open trial and then in a controlled comparative study in which ticarcillin was used. The first trial consisted of 16 patients with proven Pseudomonas aeruginosa infections, 9 of whom also received an aminoglycoside. In the second trial 28 such patients were evaluated (14 on cefsulodin 1 g 4 times daily and 14 on ticarcillin 5 g 4 times daily); all patients also received an aminoglycoside. The results of the two trials were similar in that 75% of the patients of the first trial and 86% of the second group exhibited an excellent or good response to cefsulodin. For the ticarcillin group a similar response was noted. In the first trial the sensitivity of P. aeruginosa did not change markedly, whereas one strain of the cefsulodin group became resistant in the second trial. Pharmacokinetic data were in agreement with those reported in the literature. Side effects were rare.