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.