High concentrations of particulate and dissolved lead, zinc and cadmium (total dissolved Pb, 750µg l^-1; Zn, 4300µg 1^-1; Cd, 64 µg l^-1; particulate Pb, 236 µg l^-1; Zn, 64 µg l^-1; Cd, < 1 µg l^-1) occur in seawater- like effluent from a lead-zinc smelting complex at Port Pirie. This effluent is discharged through a silled tidal channel into the offshore marine environment, where dilution by seawater eventually reduces the metal concentrations to values similar to those in near-surface seawater in Spencer Gulf, remote from centres of industrial activity (average total dissolved Pb, 0 4 µg 1^-1; Zn, < 10 μg l^-1; Cd, 0 3 µg l^-1; average particulate Pb, <0.05 µg l^-1; Zn, 0 21 µg I^-1). Precipitation of dissolved metals or conversion of originally weakly complexed dissolved metals to more strongly complexed species does not occur to a significant extent. Particulate metal concentrations are influenced by resuspension of metal-rich sediments and by uptake of dissolved cadmium by unicellular algae that grow in the tidal channel.