The influence of the zwitterionic (ampholytic) pH buffers, 3-[N-N-bis(hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO, 10 mM) and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES, 25 mM), on the toxicity of copper(II) to the marine dinoflagellate, Amphidinium carterae, was investigated. For this purpose, DIPSO or HEPES buffered complete ESAW (enrichment solution artificial seawater) growth media as well as simplified (enrichment solution deficient) buffered media were assayed in the toxicity tests, at pH 8.0. Similar assays were performed in unbuffered ESAW medium. Toxicity evaluation was based on growth changes, EC50 (effective concentration of labile copper that reduces growth rate by 50%) and esterase activity. In addition to biological parameters, electrochemical behavior of the buffers in the presence of copper(II) was used to interpret the results. Neither of the buffers were toxic to A. carterae. HEPES increased copper toxicity (synergistic effect) while DIPSO diminished it (strong antagonistic effect). HEPES does not significantly complex copper(II) and this effect was possibly due to change in kinetics of metal binding to the organisms due to its surfactant properties at 25 mM. DIPSO is a strong copper(II) complexing agent (average conditional stability constant, log K=7.3, pH 8.0, 25°C). These results suggest that DIPSO may substitute EDTA in culture media with the advantage of simultaneously buffering changes in pH and metallic cations.