The optimum conditions for the infection of Spodoptera frugiperda (Sf) 21 cells with recombinant virus were revealed. The maximum activity of Escherichinacoli ,3-galactosidase was obtained by using infected cells with 5th-passage recombinant virus. The repetition of passage for recombinant virus purification resulted in a sharp decrease in protein production, and the expressed enzyme activity with 11 th-passage virus dropped one fortieth of that with 5th-passage. The β-galactosidase productivity by Sf21 cells infected in the exponential and stationary stages of growth was almost in the same level in final product yield, indicating the absence of a “culture age” effect. An increase in cell density on initial infection, moreover, did not result in a proportional increase in the final product levels. The increase in final product was observed for multiplicity of infection (MOI) values greater than 4 in cultures infected with recombinant virus. The maximum yield of recombinant protein occurred in Sf21 cells infected with an MO1 of 10.