The novel marine terpenoid dehydrothyrsiferol (DHT) has been isolated from a Canarian red alga Laurencia viridis sp. nov (Ceramiales, Rhodomelaceae) (1). Its cytotoxicity against three human breast cancer cell lines, namely T47D, ZR-75-1, and Hs578T was examined and compared with the chemotherapeutic compound doxorubicin and the mitosis-inhibitor colchicine. Primary breast carcinomas exhibit MDR1 gene expression (3). As the investigated mammary cell lines did not exhibit rhodamine 123 efflux we proved in a P-glycoprotein (Pgp) overexpressing human epidermoid cancer cell line that the marine metabolite does not modulate Pgp mediated drug transport. Therefore, it could be used in Pgp expressing cancer cells without interference.