The ability of concentrated supernatants from Lactobacillus plantarum to produce a disruption of plasma membrane in eukaryotic and prokaryotic cells has been examined. A strain of Lact. plantarum (tolerant to acid and bile salts and resistant to several antibiotics) was used. It inhibited the growth of pathogenic Escherichia coli and L. monocytogenes. Supernatants from Lact. plantarum were concentrated by centrifugation. Either E. coli or HL-60 cells (a human promyelocytic cell line) were treated in the presence of the concentrated supernatants. The effect of concentrated supernatants from Lact. plantarum on E. coli growth demonstrated a bacteriostatic activity and a loss of cell viability measured by sytox green staining. Concentrated supernatants were capable of disturbing plasma membrane in E. coli and of promoting a cytotoxic and lyctic action on HL-60 cells and on human erythrocytes, respectively. These results suggest that Lact. plantarum release an effective compound responsible for an important effect in the disruption of E. coli plasma membrane and for a cytototoxic activity on promyelocytic leukaemia cells. This is the first in vitro study about the antimicrobial and biological activities of concentrated supernatants from Lact. plantarum.