Immobilization of insect cells using porous biomass support particles (BSPs) and production of a recombinant protein by the immobilized cells after infection with a baculovirus were investigated in a shake-flask culture. Sf9 cells were passively immobilized in reticulated polyvinyl formal (PVF) resin BSPs (2 x 2 x 2 mm cubes) with matrices of 60 mu m mean pore diameter in situ in shake-flasks. The cell density in the BSPs was over 5 x 10(7) cells/cm(3)-BSP in cultures with regular replacement of the culture medium, as estimated by 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. After infection with a recombinant baculovirus carrying the beta-galactosidase gene, immobilized cells within the BSPs showed a high specific productivity, comparable to the maximum productivity in shake-flask cultures of non-immobilized cells, as long as nutrients in the medium were not depleted. Even when immobilized cells at a high density of 5 x 107 cells/cm(3)-BSP were infected with the baculovirus, efficient beta-galactosidase production with a high specific productivity was possible by replacing the medium at appropriate intervals to avoid nutrient depletion.