Lactobacillus pentosus TV35b, isolated from the posterior fornix secretions of the vagina of a prenatal patient, produced a bacteriocin-like peptide (pentocin TV35b), which is inhibitory to Clostridium sporogenes, Cl. tyrobutyricum, Lact. curvatus, Lact. fermentum, Lact. sake, Listeria innocua, Propionibacterium acidipropionici, Propionibacterium sp. and Candida albicans. The mechanism of activity of pentocin TV35b is bactericidal, as shown by a decrease in the viable cell numbers of Lact. sake from approximately 4 x 108 to less than 10 cfu ml - 1 over a period of 4 h. Pentocin TV35b added to the growth medium of C. albicans stimulated the formation of pseudohyphae during the first 36 h, followed by a slight repression in cell growth. Production of pentocin TV35b was at its maximum towards the end of the logarithmic growth phase of strain TV35b. The peptide was purified by ammonium sulphate precipitation, followed by SP-Sepharose cation exchange chromatography. The molecular size of pentocin TV35b was estimated to be between 2.35 and 3.4 kDa, according to tricine-SDS PAGE. However, results obtained by electrospray ionization mass spectroscopy indicated that the peptide is 3930.2 Da in size. Amino acid analysis performed by using the Pico-Tag(R) method and a Nova-Pak C18 HPLC column indicated that pentocin TV35b consists of 33 amino acids with a total mass of 3929.63 Da. Pentocin TV35b is inactivated when treated with papain and Proteinase K, but remains active after incubation at pH 1-10 for 2 h at 25 degrees C, and when heat-treated for 30 min at 100 degrees C.