Three purified proteins from Bacillus thuringiensis were bioassayed in the laboratory at 27 deg C against 17 insect pests from 5 orders and one species of mite. The proteins were single gene products from B. thuringiensis subsp. kurstaki, (HD-1 (CryIA(b)) and HD-73 (CryIA(c))) and B. thuringiensis subsp. tenebrionis (CryIIIA). Of the insects tested, only the chrysomelid Leptinotarsa decemlineata was sensitive to CryIIIA protein with an LC50 of 6.5 micro g/ml. The sphingid Manduca sexta, the noctuids Trichoplusia ni, Heliothis virescens, Agrotis ipsilon, Spodoptera exigua and Heliothis zea (Helicoverpa zea) and the pyralid Ostrinia nubilalis were sensitive to the B. thuringiensis subsp. kurstaki proteins, with LC50s of 0.036, 0.09, 1, 18, 44, 10 and 37 micro g/ml, resp., for HD-73, and 0.04, 0.19, 1.7, >80, 34, 33 and 3.6 micro g/ml for HD-1. Species which were not affected by the proteins included Blattella germanica and Aedes aegypti. No synergism was observed with combinations of proteins. A larval growth inhibition assay was developed to determine the amount of B. thuringiensis subsp. kurstaki protein required to inhibit larval growth by 50% (EC50), enabling the detection of levels of HD-73 as low as 1 ng/ml. The EC50s for M. sexta, T. ni, Heliothis virescens and Helicoverpa zea were 0.008, 0.014, 0.0013 and 0.014 micro g/ml, resp.