The role of vaginal Lactobacillus as an efficient barrier against invading pathogens is of considerable interest. The purpose of the present study was to assess in vitro the ability of 20 recently identified vaginal lactobacilli to protect the vagina. In order to evaluate their significance, the antimicrobial, hemagglutination (HA) and aggregation (Agg) activities, as well as acid and hydrogen peroxide (H(2)O(2)) production, were estimated. The cell-free cultures of eight strains showed a stable antimicrobial activity after elimination of the putative effects of lactic acid and H(2)O(2). Three of the isolated vaginal lactobacilli expressed a broad spectrum of anti-bacterial activity including Gram-negative pathogens. Strains with anti-Gardnerella and anti-herpes simplex virus type 2 activities were found. All tested isolates were H(2)O(2) producers, actively acidifying the growth media to pH 3.92+/-0.04, which is presumed to neutralize sexually transmitted infection pathogens. The major part (75%) expressed an HA activity and different Agg phenotypes, estimated as important properties in the competition with invading pathogens and in host defense. These results are encouraging and prompt further research of the characterized active strains and their possible application in prophylaxis of vaginal disorders.