Recently, we reported that the synthetic Coprisin analog peptide 9-mer dimer CopA3 (consisted of all-L amino acid sequence) was designed based on a defensin-like peptide, Coprisin isolated from Copris tripartitus. The 9-mer dimer CopA3 (L-CopA3) had antibacterial activity and induced apoptosis in human leukemia cells via a caspase-independent pathway. In this study, all of amino acid sequences of L-CopA3 were modified to all D-form amino acids (D-CopA3) to develop a more effective antimicrobial peptide. We investigated whether D-CopA3 had antimicrobial activities against pathogenic microorganisms and proapoptotic effects in human leukemia cells (U937, Jurkat, and AML-2). The synthetic peptide D-CopA3 had antimicrobial activities against various pathogenic bacteria and yeast fungus with MIC values in the 4~64 microM range. Moreover, D-CopA3 caused cell growth inhibition, and increased the chromosomal DNA fragmentation and the expression of inflammatory cytokines, TNF-alpha and IL1-beta, transcripts in human leukemia cells. The all-D amino acid peptide D-CopA3 proved as effective as the L-CopA3 reported previously. These results provide the basis for developing D-CopA3 as a new antibiotic peptide.