Enhancement of productivity of a bioprocess necessitates continuous op
eration of bioreactors with high biomass concentrations than are possi
ble in conventional batch, fed-batch or continuous modes of culture. M
embrane-based cell recycle has been effectively used to maintain high
cell concentrations in bioreactors. This review compares membrane-base
d cell recycle operation with other such high density cell culture sys
tems as immobilized cell reactors and reactors with cell recycle by ce
ntrifugation or gravity sedimentation. A theoretical of production of
primary and secondary metabolites in membrane-based recycle systems is
presented. Operation of this type of system is discussed with example
s from aerobic and anaerobic fermentations.