Although guidelines exist for selecting appropriate ecological receptors for risk assessments at contaminated sites, it can be demonstrated that many of the mammals commonly evaluated are spatially irrelevant. Terrestrial risk assessments could be simplified and made more efficient, though, if mammals that are initially considered were screened for their spatial relevance. This article presents 2 simple algorithms that each demonstrate that most mammalian receptors are not spatially relevant for the overwhelming majority of hazardous waste and other contaminated sites. The algorithms use readily available and curiously overlooked spatial distribution (e.g., animal density) information and suggest that contaminated sites need to be 80 to 100 acres in size to justify the inclusion of most mammalian receptors. Given that hazardous waste sites are generally much smaller than this, many ecological risk assessments (ERA) could reasonably dispense with incorporating mammals entirely. An awareness on the part of decision makers and risk managers of the nonsuitability of many mammalian receptors evaluated in terrestrial ERAs could significantly impact the perceived need to monitor or remediate sites. This article also examines the anticipated challenges of regulators and other decision makers when entertaining the notion of a spatial relevance screen for mammals.