Influenza virus strains of different types for use as an inactivated vaccine have been successfully grown in different cell lines. Increasing titres were obtained with BHK-21/BRS, VERO and MDCK cells. Cultures in stationary flasks, in spinner cultures or in large bioreactor systems were tested and the optimal conditions were studied. MDCK cells grown in serum-free medium before and during the virus production phase were found to yield high titres in the presence of trypsin. Satisfactory results were obtained with egg-adapted strains of human and equine origin as well as with strains just isolated from human patients without any further passages in eggs or cell culture.