Studies were conducted in 30 patients with neoplastic diseases. Twelve patients received sisomicin intramuscularly at doses of 20 mg/m 2 and 40 mg/m 2 . The mean peak serum concentration occurred at 1 h and was 2.5 μg/ml and 4.0 μg/ml, respectively. Ten patients received intravenous sisomicin at doses of 30 mg/m 2 during 30-min infusion. Mean peak serum level determined at 30 min was 5.1 μg/ml. The levels gradually decreased and at 6 h was 0.6 μg/ml. The serum half-life was 160 min. Serum levels determined in eight patients who received sisomicin by continuous infusion at doses of 30 mg/m 2 every 6 h were greater than 1.4 μg/ml during the 6-h period. The urinary excretion of sisomicin during the 6-h period after intramuscular administration of 20 mg/m 2 and 40 mg/m 2 was 49 and 61%, respectively. The pharmacology of sisomicin is similar to gentamicin.