Great controversy surrounds the potential environmental effects of genetically modified crops (GMOs). Of the GM crops currently in commercial use, glyphosate-resistant (GR) soybeans have probably received the most attention from both proponents and opponents of GMOs. GR soybeans are an example of an input-substituting technological innovation. Adoption is a private decision, based on the adopter's assessment of private costs and benefits, but may also have external effects. Measuring and valuing these effects completely is not possible today. Environmental indicators are attempts to find more easily measured proxies for the external effects. We present an environmental indicator based on a standardized, well-known acute mammalian toxicity measure, the LD50 dose for rats. We use this indicator to compare an environmental effect of the use of GR genetically modified soybeans to the use of non-modified soybeans for over 1400 US Midwest farms. This indicator is superior to previous indicators used to assess environmental effects of GR soybeans that rely on adding up volumes of different herbicides. The indicator allows for consistent aggregation of a specific environmental effect across many different pesticides. Furthermore, the methodology can be used to compare environmental effects for many other types of technology choices since values are mandated for all pesticides. Our simulation results suggest that GR soybean seed technology is more environmentally friendly than non-GR technology for all farms in the dimension of acute mammalian toxicity. The effect is generally more pronounced in the South where a longer growing season makes overall weed pressure more serious and presents soybean growers with a greater variety of weed species.