Epinecidin-1 is an antimicrobial peptide present in the grouper (Epinephelus coioides). In this study, the antitumor activity of a synthetic epinecidin-1 peptide was tested. The in vitro results showed that epinecidin-1 inhibited the proliferation of human leukemia U937 cells and increased the ADP/ATP ratio after 24 h of treatment. The DNA fragmentation assay, flow cytometric assay, and caspases-3, -8, and -9 assays indicated that epinecidin-1 could induce apoptosis in U937 cells. Real-time RT-PCR results showed regular increases in tumor necrosis factor (TNF)-α after treatment with 4 μg/ml epinecidin-1 from 4 to 24 h; interleukin (IL)-10, interferon (INF)-r, p53, IL-15, and IL-6 increased after treatment with 2 μg/ml epinecidin-1 for 4-12 h. These results suggest that the epinecidn-1 inhibited U937 cells, induced apoptosis in response to cytokine production, and may have pleiotropic effects on different cells.