The effect of humic acid on accumulation of hexachlorobenzene (HCB) and hexachlorobiphenyl (HCBP) by sheepshead minnows (Cyprinodon variegatus) from two separate experiments is presented. In the first experiment, static sediment/water exposure chambers were used to determine the effect of dissolved organic carbon (DOC), in the form of terrestrial humic acid (HA), on partitioning of HCB among sediment, water, and sheepshead minnows. Sediments from three geographically different locations were used to test the effect of added HA (0, 3, and 30 mg/L) on accumulation in fish of sediment-bound HCB. Total organic carbon levels in sediment and water and residues of HCB ((14)C-labeled and unlabeled) in sediment, water, and whole-body tissue were measured. Fish/sediment ratios or accumulation relative to sediment (ARS) indicated that addition of HA did not reduce accumulation of sediment-bound toxicants. ARS ranged from 7.5 + or - 4 without added HA to 9.3 + or - 4 with 30 mg/L added HA, but were not statistically different. In a second experiment using HCBP with 0 and 30 mg/L HA and sediment from one location, HCBP concentration in water averaged 0.29 micrograms/L in non-HA tanks and 0.91 micrograms/L in the HA tanks, but both ARS ratios averaged 5.4. In conclusion, the authors believe that these tests indicate that HA does not significantly alter bioavailability of toxicants that are in equilibrium with sediment and water.