Twenty-seven patients with serious gram-negative infections were treated with ticarcillin in an average daily dosage of 237 mg/kg (range, 174 to 307 mg/kg). Ticarcillin was bactericidal for all infecting organisms in concentrations ranging from 31.2 to 125 μg/ml. Five of 8 patients (62%) with overwhelming Pseudomonas pneumonia were cured or improved, and 9 of 12 (75%) were cured of pneumonia caused by other gram-negative organisms. Of six extrapulmonary infections caused by Pseudomonas, five (83%) were cured or improved. In seven cases, the infecting organism reisolated during therapy was more resistant to ticarcillin than the primary isolate. The serum half-life of ticarcillin in three patients with renal failure was 11.2 ± 1.0 h, and during hemodialysis it decreased to 6.3 ± 1.8 h. There were two episodes of superinfection with resistant organisms. Thirteen patients (48%) manifested eosinophilia, one of whom had severe urticaria. Prolongation of bleeding time was attributable to ticarcillin in two patients. Ticarcillin appears to be effective for therapy of serious gram-negative infections in dosages 30 to 50% less than those recommended for carbenicillin.