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The phytotoxic effects of heavy metals and organic substances which can be often found in compost have been tested in the conditions commonly used for compost phytotoxicity assessment. A germination-elongation phytotoxicity test was carried out with aqueous solutions of Cu (5-50 mg L-1), Pb (5-100 mg L-1), Zn (5-100 mg L-1), acetic acid (50-300 mg L-1), propanoic acid (50-300 mg L-1), butyric acid (5-50 mg L-1), and ethanol (1-10 g L-1), and the results were compared to those of a second test in which a previous step of soaking of the seeds in the tested solutions was introduced. Three species were used: Hordeum vulgare L. (spring barley), Lolium multiflorum Lam. (Italian ryegrass) and Lepidium sativum L. (garden cress). Cress was the most sensitive species to all the substances used. The phytotoxic concentrations were found to be about 5 mg L-1 for Cu and Pb, 25 mg L-1 for Zn, 50-100 mg L-1 for the 3 organic acids, and 2 g L-1 for ethanol. The results also showed that the previous imbibition made the test more reproducible, and more sensitive since it rendered phytotoxic results at lower doses of heavy metals. Comparing the results to the usual concentrations of these substances in compost of various origins shows that organic compounds are more prone to be the cause of compost phytotoxicity in germination tests, rather than heavy metals. [ABSTRACT FROM AUTHOR]