Algal toxicity testing is not new, but only within the past few years have data from such testing been used to help set standards for allowable contamination. Early toxicity testing with marine algae used a few planktonic species with inhibition of growth as the primary endpoint. Work with vegetative growth of marine seaweeds from intertidal areas led to conclusions of insensitivity to toxicants. Within the past five years, tests with different species of microalgae and reproductive tests with marine macroalgae have come into wider use. These new species and test method endpoints have shown that plants can be more sensitive to toxicants than some of the most sensitive marine animals. It has been proposed recently that the saltwater 'Criterion Continuous Concentration' for water quality criteria documents for thallium and acenaphthene be set with reproductive data from the kelp, Laminaria saccharina; which was the most chronically sensitive species tested for both chemicals. (Copyright (c) 1993 American Society for Testing and Materials).