The in vitro cytotoxicity of the antimicrobial peptide P40 was investigated. The food grade bacteriocin nisin was also analyzed for comparison. VERO cells were treated with different concentrations (0.02-2.5 microg ml(-1)) of nisin and P40, and cell viability and plasma membrane integrity were checked by MTT, neutral red uptake (NRU), and lactate dehydrogenase (LDH) assays. In MTT and NRU assays the EC(50) to the purified peptide P40 were 0.30 and 0.51 microg ml(-1), while values found to nisin were 0.35 and 0.79 microg ml(-1), respectively. In the LDH assay, the EC(50) was 0.57 and 0.62 microg ml(-1) for P40 and nisin, respectively. The peptide P40 revealed higher hemolytical activity (19%) when compared to nisin (4.9%) at the highest concentration tested (2.5 microg ml(-1)). Relatively few studies about the cytotoxicity of antimicrobial peptides are available. The determination of the cytotoxicity of antimicrobial peptides is an essential step to warrant their safe use.