Several genes from Bacillus thuringiensis (Bt) have been genetically engineered into cotton (Gossypium spp.) plants to promote insect resistance. In one instance, the insect resistance in these engineered plants is a result of an introduced Bt gene coding for production of the Cry1Ac protein. The amount of Cry1Ac proteins produced in four cotton backcross populations and their respective recurrent Bt-containing parents was quantified using the ELISA-based Envirologix QuantiPlate Kit for Cry1Ac. Our main objective was to identify plants with two copies of the Bt gene (homozygous) versus those with one copy (heterozygous). The results clearly indicated the lack of Cry1Ac protein in our negative control while showing a continuous range of values for those plants expressing Cry1Ac.  There was no definite break among the values that would indicate heterozygotes versus homozygotes having twice the expression level of the heterozygotes. Although we were unable to identify homozygotes with this kit, the kit is easy to use and effective for selecting those plants with the highest Cry1Ac expression.