It　has　become　important　to　develop　a　simple　in　vitro　method　for　the　detection　of　environmental　carcinogen　especially　which　could　induce　severe　tumors.　In　order　to　detect　suspected　carcinogens　and　provide　experimental　evidence,　we　adopted　293　cell　transformation　systems　to　clarify　the　tumor　promotive　activity　of　Microcystin　LR　(MC-LR)　and　Benzo[a]pyrene　(B[a]P)　which　were　main　carcinogens　exist　in　drinking　water　and　air　respectively.　We　designed　a　method　for　detecting　MC-LR　and　B[a]P　by　the　use　in　vitro　cell　transformation　system.　In　this　method,　293　cells　were　cultured　in　flasks　for　21　days　during　which　test　chemicals　were　added　in　the　medium　for　7　days,　then　the　transformed　293　cells　were　tested　by　several　methods　including　soft　agar　colony　assay,　serum　requirement　assay　and　tumorigenic　activity　to　define　the　carcinogenetic　activity.　We　found　that　when　MC-LR　was　examined　by　the　method,　a　reasonable　correlation　was　observed　with　the　reported　tumor-promoting　ability　in　animal　experiments.　We　propose　that　the　293　cell　transformation　method　was　practical　and　useful　for　the　detection　of　environmental　carcinogens.