The increase of Sf9 cell diameter after infection with a recombinant baculovirus encoding VP8 protein of rotavirus can be used to predict culture productivity. A direct proportional correlation between the increase in cell size and VP8 concentration was obtained when manipulating selected medium components. Only yeast extract increased (38%) VP8 concentration, while fetal bovine serum increased (55%) the maximum cell concentration. An inexpensive and simplified culture media can thus be designed without detriment to protein yields.