Leptosins M (4), M1 (5) N (6) and N1 (7) have been isolated from a strain of Leptosphaeria sp. originally separated from the marine alga Sargassum tortile. Their absolute stereostructures have been elucidated by spectral analyses and some chemical transformations. Their NMR and NOE spectral analyses revealed that 4-7 exist in a single conformer of B type in acetone-d6. All the compounds exhibited significant cytotoxicity against cultured P388 cells. In addition, leptosin M(4) proved to exhibit significant cytotoxicity against human cancer cell lines, and to inhibit specifically two protein kinases, PTK and CaMKIII, and human topoisomerase II.