A 4 field site study was conducted in which Bollgard® II (containing 2 lepidopteran active Bt proteins: Cry1Ac and Cry2Ab) and Bollgard® (DP50B containing only Cry1Ac) cotton tissue samples were collected throughout the growing season and evaluated for total lepidopteran activity using a Heliothis virescens quantitative bioassay (Greenplate 1999). In addition, protein-specific ELISA assays were performed upon the same tissue samples to determine relative levels of both insect control proteins. Total lepidopteran activity expressed in Cry1Ac equivalents was much higher in Bollgard® II tissue samples. Overall mean lepidopteran activity levels were 3.5 as great for Bollgard® II as for Bollgard®. This relative increase in mean lepidopteran activity levels was seen at every sampling time (from 2 weeks post-pinhead square to 8 weeks post-pinhead square) and at all field sites. ELISA assays were used to estimate the levels of each toxin. The amount of Cry1Ac protein in Bollgard® II was the same as in Bollgard®. A main-effect ANOVA determined that, in addition to the Bollgard® II-Bollgard® difference, sampling time and tissue type were significant contributors to the variability among lepidopteran activity levels; although the tissue type variability can be explained by the difference between younger tissue (terminal leaf, square) and older tissue (large leaf). There was no significant difference in lepidopteran activity between terminals and squares of Bollgard® II. These data suggest that the greatest effect of the expression of Cry2Ab in Bollgard® II is likely to be greatly increased and prolonged lepidopteran activity in all tissue types, especially reproductive tissue. A summary of the overall mean lepidopteran activity for 1998 and 1999 is shown.