The capacity of several fluorenylhydroxamic acids and their acetate esters to transform Wistar rat-embryo fibroblasts grown in mass culture was investigated. N-Hydroxy-2-fluorenylacetamide, N-hydroxy-3-fluorenylacetamide, N-acetoxy-2-fluorenylacetamide, N-acetoxy-3-fluorenylacetamide, and N-hydroxy-2-fluorenylbenzamide induced morphologic alterations commonly associated with malignant transformation. Untreated controls did not change morphologically. Inoculation of carcinogen-treated cells into Wistar or Sprague-Dawley rats gave a high incidence of transplantable fibrosarcomas. Untreated cells yielded no tumors. Addition of cysteamine to rat embryo fibroblasts beFore exposure to N-hydroxy-2-fluorenylacetamide and N-hydroxy-2-fluorenylbenzamide reduced the latent period and increased the tumor yield. Preliminary studies indicated that rat embryo fibroblasts, unlike rat livers, do not activate N-hydroxy-2-fluorenylacetamide by sulfonation. This suggests that the mechanism underlying the transformation of rat embryo fibroblasts is different from that considered to account for the malignant transformation of the rat hepatic cell.