Eight cell lines derived from the insects Spodoptera frugiperda, Trichoplusia ni, Mamestra brassicae, and Estigmene acrea were evaluated for recombinant β-galac-tosidase and infectious virus production following infection with the baculovirus Autographa californica multiple nuclear polyhedrosis virus (AcMNPV). Production was assessed on a specific (per cell and per microgram of uninfected cellular protein) and on a volumetric (per milliliter) basis. Cell density was found to be an important factor in comparing the cell lines due to a density-dependent inhibition of specific protein and virus production that appeared to result from cell-cell contact. After infection of cells at low-density specific β-galactosidase production per cell would drop between β- and 6-fold in five of the eight cell lines when plated on tissue culture plates at near-confluent and confluent cell densities. The cell lines Sf 21 and Sf 9 were least sensitive to cell density. After accounting for cell density effects and differences in cell size, two cell lines, BTI Tn 5B1-4 and BTI TnM, were identified that were superior to the other cell lines, including Sf 21 and Sf 9, in β-galactosidase production. Optimal volumetric and specific β-galactosidase production from Tn 5B1-4 and TnM cells was 2-fold and 5-fold higher, respectively, in both cell lines than the optimal production from Sf 9 or Sf 21 cells. The Tn 5B1-4 cell line also had the highest viability of all the cell lines at 3 days postinfection and could be adapted to serum-free media. Specific (per microgram of cellular protein) infectious virus production between cell lines varied by less than 2-fold between cell lines after cell size differences were taken into account. No correlation was found between recombinant protein and NOV production although both were found to decrease similarly with increasing cell density.