A 5-day aquatic toxicity test based on sporulation inhibition of the green macroalga Ulva pertusa Kjellman has been developed. Optimal test conditions determined for photon irradiance, salinity and temperature were 60-200 micromol photons m(-2)s(-1), 25-35 per thousand and 15-20 degrees C, respectively. Tests were conducted by exposing U. pertusa thallus disks to a reference toxicant (sodium dodecyl sulfate; SDS), metals (Cd2+, Cu2+, Zn2+, Pb2+) and elutriates of sludge collected from nine different locations. The EC(50) values for SDS was 5.35 mg L(-1). When four heavy metals were assayed, the NOECs were highest for lead (0.625 mg L(-1)) and lowest for copper (0.031 mg L(-1)). The EC50 values showed the following toxicity rankings: Cu(2+) (0.061 mg L(-1))>Cd2+ (0.326 mg L(-1))>Zn2+ (0.738 mg L(-1))>Pb2+ (0.877 mg L(-1)). The bioassay indicated also that the sporulation endpoint could be a sensitive indicator of toxicity effects of elutriates of sludge as reflected from the NOEC values equal to or lower than the lowest concentration employed (6.25%). Sporulation was significantly inhibitied in all elutriates with the greatest and least effects observed in elutriates of sludge from industrial waste (EC50 6.78%) and filtration bed (EC50 15.0%), respectively. The results of the Spearman rank correlation analysis for EC50 data versus the concentrations of toxicants in the sludge presented a significant correlation between toxicity and four heavy metals (Cd2+, Cu2+, Pb2+, Zn2+). Introduction of the concept of toxicity unit (TU) showed that these metals were the main cause of toxicity in elutriates of at least four out of nine sludge samples. Members of the order Ulvales show a wide geographic distribution and have similar reproductive characteristics, thus making it possible to apply the present test method to other algae of this taxa, elsewhere. This novel method will be a useful tool for assessing the aquatic toxicity of a wide range of toxicants, once the respective sensitivities are demonstrated.