Neuromuscular blockades induced by clindamycin alone and withd-tubocurarine or pancuronium were examined in thein-vitroguinea pig lumbrical muscle-nerve preparation. Clindamycin, 8O-240 μ/ml, initially increased twitch tension. With higher concentrations (180–240 μg/ml) twitch tension subsequently decreased. With 15 to 20 per cent depression of twitch tension by clindamycin. neostigmine (5–20 ng/ml) or calcium (81 μg/ml) slightly but not completely antagonized the blockade. Clindamycin, 40 μg/ml, a dose that did not depress twitch tension, potentiatedd-tubocurarine- or pancuronium-induced neuromuscular blockade. Plasma concentrations of clindamycin of 10–40 μg/ml are recommended for treating serious infections. The authors conclude that the administration of clindamycin may augment nondepolarizing blockade in man, and antagonism by neostigmine and calcium may be incomplete.