The shell quality of eggs produced by northern bobwhite (Colinus Virginianus) dosed with sulfanilamide was evaluated by a compression breaking strength test and by the shell thickness measurements that are used in the U.S. Environmental Protection Agency avian reproduction test to determine the effect of a chemical substance on eggshells. The relative sensitivities of the two methods in detecting shell quality changes in sulfanilamide-treated quail were compared by relative response ratios, calculated as treatment period response divided by pretreatment values. The ratios demonstrated a clear shell weakening response and a less apparent shell thinning response. Significant differences in shell thickness of eggs from sulfanilamide-treated hens and those from control hens were not demonstrated (alpha = 0.32 and 0.34), although highly significant differences in shell strength were demonstrated (alpha <0.0001). Scanning electron micrographs of weak shells of normal thickness revealed abnormal ultrastructure, usually poorly formed mammillae. Obtaining both shell thickness and shell strength information may provide better means to evaluate a chemical's effect on shell quality.