This paper deals with the experimental results and mathematical modelling of fermentations carried out in systems with cells immobilized within a large-pore matrix. Particular attention is paid to non-stationary situations occurring in immobilized cell systems. A new model, based on the subdivision of the system into two-interconnected sections, the bulk and the support, is proposed to help the designer when selecting the most suitable start-up procedure or the best way to change the system from one steady-state condition to another. The time required to achieve steady-state conditions, as well as the fraction of active cells with respect to total immobilized biomass, is also related to the residence time for both immobilized and suspended biomass systems.