A new diffusion method employing bromocresol green for determination of blood ammonia (Ammonia CheckerR) using disposable reagent test-plates and a pocket-size colorimeter with direct readout of results was compared with an enzymatic method. The values obtained with the Ammonia Checker were slightly lower than those with the enzymatic method. Instead of arterial blood, capillary blood may be used for ammonia determination, but thorough cleansing of the fingertip used is necessary because of the high ammonia content of sweat.