The effects of humic acids (HA) on Cu uptake by the blue mussel (Mytilus edulis) were studied in chemically defined seawater. Short-term uptake by excised gills was studied and compared with whole-mussel Cu accumulation. Copper uptake in gills is not a saturable process within the time frame and concentration range tested, being a linear function of time (0-120 min) and Cu concentration (0-150 microg/L). For the whole mussel, Cu uptake is not linear with time (0-72 h) and Cu concentration (0-130 microg/L). The presence of HA (0-10 mg/L) clearly reduced Cu uptake by gills, but it did not have such an effect on whole-animal uptake. A simple complexation model obtained from voltammetric measurements was used to determine the noncomplexed (labile) fraction of Cu in the presence of HA to test the effect of speciation on Cu uptake. In most cases, Cu uptake by gills was better explained by labile Cu concentrations than by total Cu exposure concentrations, which is in agreement with the free-ion activity model. In whole-animal experiments, Cu uptake was not related to labile Cu concentrations in the presence of HA, indicating that Cu-HA complexes are at least partially available for uptake by the mussels.