Ticarcillin (BRL 2288) is a semi-synthetic penicillin which has a broad spectrum of activity, particularly against Pseudomonas aeruginosa and a variety of Enterobacteriaceae. The drug is similar in structure to carbenicillin, the former being about twice as active, and cross resistance between the two drugs is usual. Of the 17 patients with urinary tract infections who were treated with 1 g of ticarcillin intramuscularly every 6 h, 13 were considered cured after a 4-week follow-up, one did not complete the course, there was one unrelated death, and there were two treatment failures. Pain at the site of intramuscular injection was noted in five patients, the discomfort being minimized by injection into the buttock rather than the thigh. Intravenous injections did not appear to be irritating. Of the seven systemic infections, the majority of whom were treated with 5 g of ticarcillin intravenously every 6 h, there were three clinical and bacteriological cures, one case where the treatment had to be discontinued, and three cases where the efficacy of treatment was difficult to assess due to the pathological conditions present in these patients. The minimum inhibitory concentrations for the infecting organisms were determined, and the serum levels of the antibiotics were assayed. It was concluded that ticarcillin may be a useful member of the antibiotic armamentarium, particularly against Pseudomonas aeruginosa.