Mode of action of leptomycin B (LMB), a new antifungal antibiotic, was studied with Schizosaccharomyces pombe. A low concentration of LMB caused inhibition of cell division, producing elongated cells with morphologically altered nuclei and several cell plates, while it inhibited nucleic acid synthesis in intact cells at 100-fold higher concentration. Addition of LMB during G2 phase in synchronous culture blocked following events in cell cycle. Analysis of the effect of LMB on cdc mutants suggested the antibiotic inhibited some specific step, possibly in M phase just prior to nuclear division.