Laboratory studies were conducted using ELISA techniques to quantify toxin levels among plant parts of cotton expressing one and two insecticidal proteins of Bacillus thuringiensis. For Cry1Ac, consistent patterns were seen for both 2000 and 2001 among all plant parts. The highest concentrations were seen in lower canopy leaves, at 3.6 ppm. Terminal leaves contained 2.4 ppm for Cry1Ac. The lowest concentration of Cry1Ac in both single and dual-toxin varieties was found in internal boll contents, at 1.5 ppm. Pink flower parts contained less toxin than white flower parts, indicating a dramatic reduction in toxin levels over a period of 1-2 d. For Cry2Ab, a different pattern of expression among plant parts was noted between 2000 and 2001. For tissue collected in 2000, Cry2 toxin levels were higher in old leaves compared to terminal leaves (as with Cry1Ac), but the highest concentration of all plant parts was seen in white stamens (26.2 ppm). The lowest concentration of Cry2 in 2000 was found in 10-14 day-old boll walls (10.0 ppm). As in 2000, lower canopy leaves contained more Cry2 toxin in 2001 tissue than did terminal leaf tissue. Additionally, lower canopy leaves contained the highest concentration of toxin among all plant parts in 2001. The lowest concentration of Cry2 in 2001 plant tissue was seen in pink petals (5.4 ppm). Except for petal tissue collected in 2000, flower parts exhibited the same pattern for Cry2Ab as with Cry1Ac, regarding the reduction of toxin expression during the transformation period from fresh white flower to aging pink flower.