Secondary cultures of Syrian hamster embryonic fibroblasts were tested for transformation and neoplastic properties after exposure in vitro to furylfuramine (AF-2) and other nitrofurans. Typical morphological transformation was seen in five of six cultures between 30 and 186 days following treatment with 5-10X10(6) M AF-2 for 24 h. Transformation was seen in only one of four cultures 145 days after treatment for 6 h with AF-2. Treatment with 5-10X10(6) M NMF (5-nitro-2-methylfuran) for 24 h also induced transformation after 50 and 118 days in two cultures. In contrast, untreated cultures and cultures treated with 5-10X10(-6) M NFT [4-(5-nitro-2-furyl)thiazole] for 24 h were not transformed within 200 days. Three of the six lines transformed by AF-2 and both lines transformed by NMF also became tumorigenic 7-24 days after morphologic transformation. The other three transformed lines produced nodules which regressed within a few weeks of transplantation. Untreated and treated non-transformed lines did not produce tumors during an observation period of 6 months. The tumors were classified as fibrosarcomas. The ability to form colonies in soft agar was acquired by only one tumorigenic line.