Anthers and pollen from corn, Zea mays L., expressing Bacillus thuringiensis (Bt)-derived protein frequently fall onto common milkweed, Asclepias syriaca L., growing in and near corn fields. Previous studies have shown that, alone, Bt anthers do not pose a significant risk to the monarch butterfly, Danaus plexippus L. To examine how exposure to a combination of Bt anthers and pollen affects larval fitness and behavior, three studies were conducted. A laboratory study using severed leaves in petri dishes and two studies with caged milkweed plants (tropical milkweed, Asclepias curassavica L., and common milkweed) in an environmentally controlled rearing room. In the petri dish bioassay, additive effects of Bt anthers and pollen were detected. The data suggest that the additive effects may be attributed to behavioral changes in larval feeding such as detecting and avoiding Bt anthers. An additive effect also was seen in both cage studies. In the common milkweed cage study, larvae exposed to Bt anthers and pollen took 1.8 d longer to develop and pupae weighed 6.4% less than those exposed to non-Bt anthers and pollen. These effects are similar to those found in a previous study with naturally deposited levels of Bt anthers and pollen, even though the anther levels we tested were two to three times greater. Despite these effects, when put into the context of risk, Bt corn is not likely to pose a significant risk to the monarch butterfly population in North America.