The future application of immobilized microorganism techniques will depend on the development of systems which are technologically applicable on an industrial scale. These techniques must permit high microbial concentrations and must allow mass transfer to take place with low diffusional limitations. In addition, the mechanical separation of the immobilized microorganisms must be achieved economically. Kappa-carrageenan was used as an ionotropic gel carrier for the immobilization of Lactobacillus bulgaricus (ATCC 11842), and the effects of gel stability on productivity and the rate of product formation were investigated. kappa-carrageenan gels had higher mechanical and chemical stability than alginate gels. The storage stability of microorganisms immobilized in kappa-carrageenan was good enough to retain biocatalytic activity during prolonged storage at 4 degrees C.