Clones isolated from the heterogenous Spodoptera frugiperda IPLB-Sf-21-AE and Mamestra brassiace IZD-Mb-O503 insect cell populations have been compared in regards to their synthesis of recombinant Escherichia coli beta-galactosidase (beta-gal), human tissue plasminogen activator (t-PA), and human kidney renin. Significant differences were found among the clones in their expression levels of these proteins. In addition, the expression levels of t-PA and renin were considerably less than that of beta-gal. Clone BC5 was the best producer of t-PA and renin among the Mb-O503 clones, whereas clone BB5 was the best producer of beta-gal. The Sf-9 cell line and clone 2 were the best producers of t-PA and renin, respectively, among the Sf-21 clones. The Mb-O503 clones generally were better producers of t-PA than the Sf-21 clones, with the BC5 clone producing twice the levels of any other Mb-O503 or Sf-21 clone. The Sf-21 clones were generally better renin producers; the Mb-O503 clones were generally found to have beta-gal expression levels superior to those reported previously for the Sf-21 clones.