In textbook treatments of enzyme inhibition kinetics, adjustment of the initial inhibitor concentration for inhibitor bound to enzyme is often neglected. For example, in graphical plots such as the Dixon plot for estimation of an inhibition constant, the initial concentration of inhibitor is usually plotted instead of the true inhibitor concentration. Justification for this approximation should be presented to students, because in certain conditions it can be inaccurate. (Contains 3 figures and 1 table.)