Abstract: The malignant transformation of 3T3 cells induced by microcystin-LR (MC-LR) was studied. 3T3 cells were cultured with MC-LR for three weeks. The biological characteristic changes of 3T3 cells induced by MC-LR were evaluated by cell growth assay, serum requirement assay and clone forming in soft agarose. The transformed cells were also inoculated into the hypophloeodal of right back of SCID mice. Results Under the induction of MC-LR, the modality of 3T3 cells changed distinctly and its contact control character disappeared. Furthermore, its growth speed increased while its serum requirement decrease. The control 3T3 cells could not develop tumor in SCID mice. After culturing with MC-LR, the 3T3 cells could develop tumors in SCID mice and come into being clone in soft agarose. Malignant transformation of the 3T3 cells can be induced by MC-LR.