High levels of ammonia in sediment toxicity tests can potentially confound test results. At issue is whether the observed toxicity is due to elevated ammonia or the presence of more persistent anthropogenic contaminants. To evaluate the risk of ammonia toxicity, information on (1) the exposure–response relationship and (2) the probability of exposure are needed. Preliminary exposure–response data were obtained for two different sieved size classes of the estuarine amphipod Leptocheirus plumulosus representing juveniles (425–600 μm) and subadults (not reproductively active) (600–1,000 μm) in 4-d water-only exposures with ammonia (NH4Cl). Resulting LC50 values were 44 (juvenile) and 89 mg NH3-N/L (subadult). Results from 4-d tests were used to establish exposure concentrations for two spiked-sediment studies: (1) a 10-d static exposure starting with subadults measuring survival and growth and (2) a 28-d exposure with daily renewal of overlying water starting with juveniles and measuring survival, growth, and reproduction. Risk of ammonia toxicity to L. plumulosus during sediment toxicity tests was characterized via a Monte Carlo simulation using the exposure–response curves from laboratory spiked-sediment studies and published porewater ammonia values for 322 marine sediments. Results indicate a significant potential for ammonia induced interference in the 10-d test (e.g., 2–18% probability of significant mortality due to ammonia alone). Risk in 28-d tests appears to be minimal due to rapid dissipation of ammonia via renewal of overlying water.