Leptolstatin, a product from Streptomyces sp. SAM1595, is a gap phase-specific inhibitor of the mammalian cell cycle. Physico-chemical properties and spectrometric analyses showed that the structure of leptolstatin is (2Z,6E,8Z,12E,14E,22E)-19,24-dihydro xy-8,10,14,16,18,20,22-heptamethyl-17-oxo-2 , 6,8,12,14,22-tetracosahexaen-5-olide.