The effect of hexafluorenium 0.3 mg/kg on the neuromuscular block of suxamethonium 0.1 mg/kg in a child with homozygote atypical plasma cholinesterase activity and in one of his normal brothers has been compared. In the normal child, hexafluorenium produced a marked potentiation of suxamethonium block, while partially antagonizing the block in the atypical homozygote child. In both situations, the hexafluorenium-suxamethonium block appeared to be antidepolarizing in nature as indicated by tetanic fade and post-tetanic facilitation. It is concluded that, in man, hexafluorenium can modify the action of suxamethonium by two different mechanisms. In patients with normal plasma cholinesterase activity, the anti-esterase effect predominates and results in marked potentiation of suxamethonium block, while in atypical homozygotes a direct antidepolarizing action at the neuromuscular junction may result in a diminished response.