The frequency and severity of side effects, above all, phlebitis, associated with an intravenous use of cephapirin (CEPR) or cephalothin (CET) was compared in 69 patients with infections. Two grams of each drug were administered intravenously twice a day with a 21-G vein needle in one of the two arms of the patients. CEPR was administered to 32 patients, and CET to 37 patients respectively. After treatment, the status of the veins was checked, and laboratory findings and other side effects were evaluated daily. Each drug appeared to be equally efficacious in the treatment of infections. The administration of CEPR was associated with a slightly lower rate of phlebitis and other side effects, but the difference between the 2 drugs was not significant (0.05 less than P less than 0.10). Phlebitis was observed in 1 patient (3.1%) of CEPR group and in 3 patients (8.1%) of CET group. Side effects, including phlebitis, were observed in 4 patients (12.5%) of CEPR group and in 12 patients (32.4%) of CET group. In CET group, drug exanthema (3 cases), drug fever (3 cases), and abnormalities in liver function (4 cases) were observed. These findings, together with the results of other reports, suggest that CEPR is a safe and useful drug in the treatment of infection as compared with CET.