The individual and combined toxicity of dissolved mercury, copper, lead and cadmium has been investigated by using the Parcentrotus lividus sea-urchin embryo-larval bioassay. Embryogenesis success and early larval growth have been recorded after incubation of fertilised eggs in seawater, both with single metals and binary combinations of Hg with every other metal. For individual metals the ranking of toxicity was Hg > Cu > Pb > Cd, with EC50 values of 21.9, 66.8, 509 and 9240 micrograms/l, respectively. Lowest observed effect concentrations (LOEC) for early larval growth were approximately three times lower than the EC50 values for Hg, Cu and Pb, and more than two orders of magnitude lower for Cd, emphasizing the danger of underestimating toxicity when only lethal effects are recorded. Marking & Dawson's additive indices ranged from 0.10 to 0.19, indicating additive effects with a slight trend to synergism, which was statistically significant for the Hg-Pb combination only. Hayes' additive indices were within the margins considered acceptable to describe additive interactions.