Despite the heterogeneity of natural organic matter (NOM) in the aquatic environment, current models that predict metal toxicity to aquatic biota treat these important metal-complexing agents in a homogeneous manner. In this investigation, the ability of 11 commercial and naturally isolated NOM samples to ameliorate silver toxicity to the freshwater crustacean Daphnia magna was examined. The commercially available Aldrich humic acid (AHA) increased the 48-h median lethal concentration for daphnid neonates from nominally NOM-free levels of 0.29 to 3.80 μg/L (at 6.9 mg C/L) in a concentration-dependent manner. Three of the tested samples exhibited similar protective effects, but the additional seven NOM samples displayed significantly stronger ameliorative actions. In fact, four samples of both commercial and naturally isolated origin demonstrated greater than fourfold increases in protection compared to that of AHA. Additional investigations showed that increased silver-AHA equilibration time resulted in decreased toxicity. Increased equilibration time also decreased whole-body silver accumulation at NOM levels less than 1 mg C/L. The present results suggest that heterogeneity of NOM and silver-NOM equilibration time will have to be accounted for in future models of silver toxicity to D. magna and that laboratory toxicity testing using NOM and metals should account for the effects of metal-NOM equilibration time.