The pharmacokinetics of intravenously administered cefotiam (CTM), using a two-compartment or three-compartment open model, have been investigated in patients undergoing thoracic surgery. Patients (Group 1) given 1 hour drip infusion of 1 g of CTM, had the peak serum level (32.8 micrograms/ml) at 1 hour, and the peak pleural effusion level (8.3 micrograms/ml) was achieved at 2.08 hours. Patients (Group 2) given an one-shot intravenous injection of 1 g of CTM, had the maximum pleural effusion concentration (8.35 micrograms/ml) at 2.67 hours. Patients (Group 3) given 1 hour drip infusion of 1 g of CTM, had the mean concentration (2.3--2.5 micrograms/g) in the pleural tissue for 2 to 3.5 hours. Clinical study comprising 20 patients was performed to evaluate the effects of CTM as a prophylactic antimicrobial agent in the thoracic surgery. Patients received intravenous administration of 4 g/day of CTM for 7--10 days. Each patients was evaluated daily for fever, signs of allergic reaction, and wound infection and so on. No infections occurred in these thoracic surgery except 1, and no serious side effects was observed in this study.