The phytotoxicity of bulk and nanoparticle Cu and Ag was directly compared. NP Ag reduced biomass and transpiration by 66–84% when compared with bulk Ag. The Ag ion concentration was 4.4–10‐times greater in NP than bulk particle solutions. The Cu ion concentration was 1.4–4.4‐times greater in bulk than NP amended solutions. Humic acid (50 mg/L) decreased the ion content of bulk Cu solution by 38–42% but increased ion Cu content of NP solutions by 1.4–2.9 times. Bulk and NP Cu were highly phytotoxic; growth and transpiration were reduced by 60–70% relative to untreated controls. NP Cu phytotoxicity was unaffected by solution type, but humic acid (50 mg/L) completely alleviated phytotoxicity caused by bulk Cu. The data demonstrate differential toxicity of Ag NP relative to bulk Ag. The finding that humic acid and solution chemistry differentially impact bulk and NP behavior highlights the importance of evaluating nanoparticles under environmentally relevant conditions. © 2010 Wiley Periodicals, Inc. Environ Toxicol, 2012.