The food dye erythrosine (Erythrosin B; FD & C No. 3) was applied to isolated neuromuscular synapses in the frog, and its effects on the spontaneous quantal release of acetylcholine were examined with electrophysiological techniques. At concentrations of 10 muM or greater this anionic dye produced an irreversible, dose-dependent increase in neurotransmitter release. This increase did not depend on the presence of calcium ions in the bathing medium. These increase did not depend on the presence of calcium ions in the bathing medium. These results suggest that erythrosine might prove a useful pharmacological tool for studying the process of transmitter release, but that its use as a food additive should be reexamined.