Saccharomyces cerevisiae cells were immobilised in calcium alginate fibres and used in a reactor as a source of alcohol dehydrogenase for the NAD(+) to NADH reaction. Kinetic parameters were established for both the free enzyme and the fibre reactor. Detailed calculations for the free enzyme studies established the superiority of the Elmore-Kingston-Shields computer calculation of the initial rates. There was little difference between the Cornish-Bowden and the Hanes methods for subsequent generation of the kinetic parameters. Initial reaction rates that were obtained for the free enzyme gave kinetic parameters which were 65-80% below those obtained in the bioreactor. It is apparent that the diffusion limited the rate of reaction in the immobilised system. The bioreactor operated at high conversions at relatively low inlet concentrations of substrates. As the substrate concentration was raised the percentage conversion fell even though the amount of product produced overall rose substantially.