NAChRs heterologously expressid in human cells after transient transfection with α₃β₄α₅ or α₃β₄ subunit cDNAs exhibited similar sensitivities to antogonists and comparable functional channel profiles. However, the sum of two Hill equations was required for best fitting the ACh dose-current response curves after co-expression of α₅ α₃ and β₄ subunits. One component was comparable to that obtained in α₃β₄-transfected cells, while the additional component, putatively attributed to an α₃β₄α₅ nAChR population, showed a Hill coeeficient > 2 and a nine-fold greater half-maximal ACh concentration (EC₅₀). These results suggest that the α₅ subunit participates in the assembly of α₃β₄α₅ nAChRs compelxes in human cells, adding a new member to the family of neuronal nicotinic receptors.