An automated procedure is described for the measurement of ATP. This method, based on the linear luminescence response of firefly extracts added to ATP, is rapid, reproducible, and allows a relatively large number of samples to be run (up to 35/hr.). A procedure is also given for the preparation of the luciferin-luciferase system from dried firefly lanterns. Sample volumes required are small, and reading error is about ± 3%. Uniform timing of the addition of reagents and reading of light emission intensity for each sample are insured. Variables investigated include dilution of firefly extract, alterations in time after mixing sample and enzyme, linearity, cation, anion, and ADP and AMP interference.