Intracellular foreign protein (beta-galactosidase) expression in recombinant CHO cell lines in continuous culture was analyzed by developing a mathematical model that includes the effects of metabolic burden and cell cycle dependence of intracellular foreign protein expression. This combined growth kinetic and cell cycle model, assuming S- or G1-phase-dependent expression, was stimulated to predict productivity on a single-cell and culture-volume basis in continuous cultures. In the case of S-phase-dependent expression, the intracellular foreign protein level increases monotonically, but in the case of G1-phase-dependent expression it decreases monotonically with increasing dilution rate. Also, the trends of foreign protein concentration in the culture volume differ significantly between S- and G1-dependent expression kinetics. Thus, the cell cycle dependency of foreign protein expression should be included in process optimization concepts and operating strategies of continuous bioreactors.