Recognition of butyrylcholinesterase (EC 3.1.1.8) variants in human serum is essential to identify patients who may be susceptible to a prolonged reaction of suxamethonium and mivacurium, short-acting muscle relaxants. Thus they can be given appropriate advice along with their relatives who may be similarly affected. Therefore, Cholinesterase Unit for detection of individuals, carriers of inherited suxamethonium sensitive butyrylcholinesterase variants was established at the Institute for Clinical Chemistry of the Clinical Hospital >>Merkur<<, Zagreb, Croatia. A study was conducted on sera from patients referred to the Unit. Butyrylcholinesterase variants were determined by measuring the enzyme activity and inhibition by specific inhibitors in the sera of 384 patients and of the members of seven families. Cholinesterase Unit issued >>Warning Cards<< to the carriers of inherited serum butyrylcholinesterase variants in order to avoid prolonged apnea that suxamethonium might cause.