Attempts to apply traditional techniques of enzyme kinetic analysis (Lineweaver-Burk and Eadie-Hofstee plots) to enzymes compartmentalized by the membranes of cells, organelles, or vesicular membrane fragments will generally lead to incorrect estimates of the kinetic constants of the enzymes and incorrect conclusions about the mechanism of reaction. The error is the consequence of concentration differences, arising through the reaction process, between the solution outside the cell or oi'ganelle membrane and the solution in the interior where the reaction is taking place. The use of enzymological plots to interpret data from membrane transport studies, for example, may therefore be misleading. The specific form of the modifications to be expected in these plots for the most common types of reaction mechanisms is presented in the text in graphical form, with the algebraic expressions summarized in an appendix. The plots show curvatures which may be incorrectly interpreted as implying co-operative kinetics when, in fact, they are quite pedestrian. In some simple cases, there exist alternative ways of plotting the data which permit the correct evaluation of kinetic constants and distinction among mechanisms. The effect of cellular compartmentation on inhibition studies is also described.