Continuous production of lactic acid from lactose has been carried out in a stirred-tank reactor with non-growing Lactobacillus helveticus entrapped in calcium alginate beads, A considerably longer operation half-life was obtained in a continuously operated reactor than in a batch-operated reactor. It is possible to simulate the action of entrapped non-growing cells on the basis of information from diffusion and kinetic experiments with suspended free cells, The simulation fit the experimental data over a broad range of substrate concentrations if the specific lactic acid production rate, q(P), was used as a variable parameter in the model. The dynamic mathematical model used is divided into three parts : the reactor model, which describes the mass balance in a continuously operated stirred-tank reactor with immobilized biomass, the mass-transfer model including both external diffusion and internal mass transfer, and the kinetic model for uptake of substrate on the basis of a Michaelis-Menten-type mechanism. From kinetic data obtained for free biomass experiments it was found. with the use of non-linear parameter estimation techniques, that the conversion rate of lactose by L. helveticus followed a Michaelis-Menten-type mechanism with Ks at half-saturation = 0,22 +/- 0.01 g/l. The maximum specific lactose uptake rate Sor growing cells, q(S,max) varied between 4.32 +/- 0.02 g lactose g cells(-1) h(-1) and 4.89 +/- 0.02 g lactose g cells(-1) h(-1), The initial specific lactose uptake rate for non-growing cells, q(S,0), was found to be approximately 40% of the maximum specific lactose uptake rate for growing cells.