The possibility of determining the Michaelis constant of the irreversible deamination of adenosine to inosine by adenosine deaminase, using capillary electrophoresis, was investigated. This paper describes the use of electrophoretically mediated microanalysis (EMMA) as the technique for carrying out the assay. Initial reaction velocities of the enzymatic reaction were estimated from the peak area of inosine, and the Michaelis constant was calculated according to the Lineweaver-Burk equation. The result (Km = 5.3 x 10(-5) M +/- 8 x 10(-6) M) was consistent with previously reported values. Using the present method, a total amount of as few as 1.2 fmole of enzyme and 9.2 ng of substrate were injected in the capillary for the construction of a Michaelis Menten curve (seven concentrations of substrate, each concentration analyzed in triplicate), which is far smaller than the quantities required in conventional methods.