The eight dominant discharges of land-based waste entering Izmit Bay, Turkey, as a result of its burgeoning development are characterized here. Chemical analyses of the discharges are described and their toxicity has been quantified as a result of short term C-14 algal bioassays. Predictions of toxicity distributions in the upper layer of the bay are obtained by combining the inflow rates of the eight discharges and their quantified toxicities with a two-dimensional dispersion equation. Uptake by phytoplankton appears to be the dominant mechanism reducing toxicity in the bay. The second mechanism reducing the toxicity of the waste is dispersion by wind-induced current. The results show that although most of the factories contributing to the discharges have treatment plants, the treatment is insufficient to eliminate toxicity.