The effects of Ni2+, Co2+, and Mn2+ on human serum butyrylcholinesterase (BChE, acylcholine acylhydrolase E.C. 126.96.36.199) were investigated in this study. Inhibition kinetics of BChE were studied using butyrylthiocholine (BTCh) as substrate. The "1/v" versus "1/[BTCh]" plots in the absence (control plot) and in the presence of the metal ions intersected above 1/[BTCh]-axis for all trace elements. In addition, when the concentrations of the cations were increased at 4 mM BTCh, velocities decreased and drove to zero at high concentrations of the trace elements. These results demonstrate that Ni2+, Co2+, and Mn2+ are linear mixed-type inhibitors of BChE. alphaK(i) values have been determined as 53.20 mM, 152.25 mM, and 190.24 mM for Ni2+, Mn2+, and Co2+, respectively, by using nonlinear regression analysis. From the comparison of alphaK(i) values of the trace elements, it can be said that BChE has more affinty to binding Ni2+ than Co2+ and Mn2+.