The enzymes cholinesterase and urease were mixed individually with gelatin and immobilized onto the inside surface of glass capillary tubes. After the gelatin-enzyme mixture had dried, water samples containing various enzyme inhibiting test chemicals were pumped through the tubes to enable interaction to proceed between the foreign agents and the enzymes. Substrate for each enzyme was later passed through the tubes and any difference in relative enzyme activity determined. This procedure may be useful for health and environmental studies, particularly in the rapid testing for micro- and nanomolar quantities of enzyme-inhibiting pollutants in natural waters and complex effluents.