The influence of humic acid (HA) on the toxicity and bioavailability of two forms of trivalent chromium (chromic chloride and chrome lignosulfonate) was evaluated using a common freshwater invertebrate, Daphnia pulex. With both compounds, the 50 mg/liter HA significantly decreased toxicity at all time points examined. The remaining two HA concentrations, 0.5 and 5 mg/liter, either had no influence or decreased the toxicity of the compounds. Humic acid appeared to have no influence on the bioavailability of chrome lignosulfonate. However, for chromic chloride, 5 and 50 mg/liter HA decreased the percentage free chromium at all time points examined.