The oxidn. of pyrite in a mine tailings heap near Baia Mare, Romania, was studied to clarify the contribution of acid drainage to the death of vegetation on the eastern slope of the tailings. Where vegetation died, pyrite oxidn. was detected as deep as 1 m, as indicated by the brownish color of the samples. At these sites,71 Thiobacillus ferrooxidans-like bacteria were present with cell counts of 103 g, and Thiobacillus thiooxidans-like bacteria occurred with cell counts of 10 g. On the western slope, where vegetation thrived, cell counts of <10 g were measured. Correspondingly, leaching activity and concns. of pyrite oxidn. products such as sulfate and elemental sulfur were markedly higher on the eastern slope than on the western slope. These differences were mainly a result of the high acid neutralization potential in the material from the western slope, which kept the pH in a neutral range, allowing for plant growth. In contrast, on the eastern slope, with its low acid neutralization potential, the pH dropped to 3-4, increasing the soly. of phytotoxic elements. The death of the vegetation could result from the lowered pH and the increased amt. of toxic elements, or, most likely, a combination of these factors.