Echinomycin-7 is an echinomycin derivative, Smethylated sulfonium perchlorate of echinomycin. We studied the in vitro cytotoxicity and in vivo antitumor activity of echinomycin-7 against P388 leukemia cells and compared the results with echinomycin. With respect to the cytotoxic effects, echinomycin-7 had cell line-dependent $IC_{50}$ values while echinomycin had similar values to several tumor cell lines. Also, in vivo antitumor activities were observed in tumor-bearing mice treated with both agents, which showed that echinomycin-7 had a broad therapeutic dose range. We also observed the apoptosis on leukemia cells treated with echinomycin-7 which exihibited the ladder pattern of DNA on electrophoresis. In addition to apoptosis, echinomycin-7 arrested $G_1/S$ phases of the cell cycle at the same time. We then examined the signaling pathway of echinomycin-7-induced apoptosis and showed that ERK of the MAP kinase family was activated and translocated into the nucleus by echinomycin-7 stimulation. This study suggests that echinomycin-7 acts as an antitumor agent through in vitro cytotoxicity and has in vivo antitumor activity against leukemia cells, and that the echinomycin-7- induced apoptosis might involve signal transduction via MAP kinases.