The interaction of three new non-depolarizing neuromuscular blocking agents--atracurium, vecuronium and Duador--on human red cell acetylcholinesterase (AChE; EC 3.1.1.7) and human plasma butyrylcholinesterase (BuChE; EC 3.1.1.8) was investigated. The binding of these neuromuscular blockers to human plasma proteins (protein binding) was also studied with a new method not requiring dialysis. For sake of comparison the protein binding and the interaction of tubocurarine and pancuronium with AChE and BuChE were observed also. None of the drugs studied was a substrate of AChE or BuChE. All had a relatively weak inhibitory effect on AChE (I50 greater than 10(-5) mol litre-1 in assay systems containing 5% haemolysed red cells). Of the three new neuromuscular blockers, vecuronium and Duador were relatively potent inhibitors of BuChE (I50 less than 10(-5) mol litre-1 in assay systems containing 5% plasma), but less potent than pancuronium (I50 = 6.1 X 10(-8) mol litre-1). All neuromuscular blocking drugs tested, especially vecuronium and pancuronium, were strongly (77-91%) bound to plasma proteins.