An antifungal antibiotic, leptomycin B (LMB), which induced cell elongation of fission yeast, Schizosaccharomyces pombe, was found to be a unique inhibitor of the cell cycle of mammalian and fission yeast cells. Proliferation of rat 3Y1 fibroblasts was reversibly blocked by LMB in both the G1 and G2 phases and the treated cells were presumably introduced into the resting state (GO). After removal of LMB, proliferative tetraploid cells were produced from the cells which had been arrested by LMB at the G2 phase, as a result of DNA replication without passage through the M phase. LMB also inhibited the proliferation of S. pombe in both the G1 and G2 phases. These results suggest that the molecular target of LMB is one of the components necessary for progression of both G1 and G2 in the eukaryotic cell cycle.