Neuromuscular blocking properties of ribostamycin (1gm), dibecacin (100mg) and tobramycin (60mg) were studied in man during anesthesia and surgery by observing the effects of these antibiotics on muscle twitch tension. These drugs alone did not show any neuromuscular blocking action in those therapeutic doses. However, during the recovery phase of d-tubocurarine block the intravenous administration of 1gm of ribostamycin caused a fairly rapid decrease in twitch tension. Tobramycin 60mg did not show any remarkable effect, but dibecacin 100mg produced a slight potentiating effect on the action of d-tubocurarine. The enhancement of the action of dtubocurarine was antagonized promptly by edrophonium and more slowly by calcium.