A flow system was developed, using a Technicon AutoAnalyzer, that is readily adaptable to a range of enzyme assays. The system includes lines for pumping substrate, cofactor, buffer and enzyme and for generating linear gradients. By using a variable-speed proportioning pump the incubation time may be continuously varied, and the system also allows for continuous variation in the pH, substrate or cofactor concentration, incubation temperature and enzyme concentration. A FORTRAN V program was written that uses instrument calibrations to calculate the flow rates in the individual lines, the incubation time and the characteristics of the gradient used. The computer then prints out instructions for preparation of reagents to give a required reaction mixture, weighing sheets for stock solutions and the results of the assay in international units in suitable tables and graphs. The flow system and computer program are designed to facilitate the automation of manual assays. A detailed example is given of the use of the system [the assay of three dehydrogenases in yeast: l(+)-lactate dehydrogenase, d(−)-lactate dehydrogenase and succinate dehydrogenase], and the general applications of the method are discussed. The program has been deposited as Supplementary Publication no. SUP 50002 at the National Library for Science and Technology, Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies may be obtained on the terms indicated in Biochem. J. (1970), 116, 7.