Laboratory and clinical studies have been conducted to evaluate clinical usefulness of mezlocillin, a recently introduced broad-spectrum, semi-synthetic penicillin, with the results leading to the following conclusions:<BR>1. Antibacterial activity<BR>The antibacterial activity of mezlocillin was assessed against clinical isolates of E. coli, Klebsiella, Pseudomonas aeruginosa, Enterobacter cloacae, Serratia marcescens, Proteus mirabilis, Proteus vulgaris and Staphylococcus aureus in comparison with carbenicillin (CBPC) and ticarcillin (TIPC). The data obtained showed no significant difference in activity among the three drugs although Mezlocillin proved slightly more active than the other two antibiotic against some of the bacterial species.<BR>2. Absorption and excretion<BR>The average serum antibiotic level in three subjects was determined to be 130.7 μg/ml, 80.7 μg/ml, 44.0 μg/ ml, 13.2 μg/ml, 2.1 μg/ml and traces at 1/4, 1/2, 1, 2, 4 and 6 hours, respectively, after an intravenous dose of 2.0 g. The intravenously administered mezlocillin was recovered by 59.1% in the urine in 6 hours following injection.<BR>Intravenous drip infusion of mezlocillin in a dose of 6.0 g yielded serum concentrations of 130 μg/ml, 300 μg/ml, 640 μg/ml, 24 μg/ml and 15 μg/ml at 1/2, 1, 2 (conclusion of infusion), 4 and 6 hours, respectively, after the initiation of infusion.<BR>Parallel sputum assays showed values: 4.0 μg/ml and 10.2 μg/ml at 2 and 6 hours, respectively.<BR>The serum antibiotic concentration was found to be 50.0 μg/ml at 1 hour and 48.0 μg/ml at 2 hours following the start of drip infusion of 2.0 g mezlocillin.<BR>3. Clinical results<BR>Clinical trials of mezlocillin were performed in 9 patients, 5 with RTI and 4 with UTI.<BR>The treatment was moderately effective in 2, slightly effective in 1 and ineffective in 2 of the 5 cases of RTI, and markedly effective in 2, moderately effective in 1 and slightly effective in one of the 4 cases of UTI.<BR>The patients with RTI received the drug in doses as high as not less than 6.0 g q. d. but the treatment failed to produce gratifying therapeutic responses, whereas in UTI, mezlocillin therapy was remarkably effective (moderate to marked improvement in 3 cases).<BR>4. Side-effects<BR>Except for skin eruption observed in one case, no other adverse reaction or side effects were reported or noticed.<BR>Monitoring of clinical laboratory test did not show any abnormal values.<BR>From the results of this investigation is concluded that mezolcillin is as effective as CBPC and TIPC in treating infections with gram-negative bacilli, including Pseudomonas aeruginosa.