Spodoptera frugiperda Sf9 cells were cultivated in serum-containing (Grace and TC-100 + 10% FCS) and serum-free (Excell 401) medium in batch and continuous cultures. The cells, infected by recombinant Autographa californica, formed beta-galactosidase which was only partly secreted. The total cell concentration was monitored on-line by measuring the optical density (OD) and the viable cell concentration in situ by measuring the intensity of the background-corrected culture fluorescence (FI). By plotting 1/FI vs. 1/OD, the phases of cell growth, virus infection, and expression of virus genome, and the lysis of the cells were identified. Extracellular lactate dehydrogenase (LDH) and intracellular beta-glucosidase were monitored on-line. Serum-containing media yielded better productivity than the serum-free medium. In batch culture, cell concentrations up to 4 x 10(6) cells ml(-1), a specific growth rate of mu = 0.0288 h(-1), a doubling time of t(D) = 24 h, and with the baculovirus vector, specific beta-glucosidase activities of 3.0-40 U 10(-6) cells which increased above 100 U 10(-6) cells by the end of the batch phase, were observed. During a 626 h continuous culture, steady-state cell concentrations of 10(6) cells ml(-1) and specific beta-galactosidase activities of 15-50 U 10(6) cells were obtained. DNA content, determined by the use of a laser flow cytometer, increased during the virus infection.