For some time a bactericidal activity is recognized by air ionization. Recently it has been demonstrated that while the bactericidal effect of the positive ions is due to physical factors, the negative ions operate either by physical or chemical effects. This depends on the presence of oxygen: negative air ionization consists mainly of oxygen ions with a strong oxidizing effect. Beneficial effects of negative air ionization on human health have been recently demonstrated in several studies. In this research the bactericidal effect of the negative air ionization on some strains of Gram+ and Gram- bacteria has been estimated. Trials with Escherichia coli ATCC and Staphylococcus aureus ATCC strains have been carried out using a point-acting tester as generator of negative oxygen ions. The ion concentration was estimated in 8-10(6) ion/cm3 air at a distance from the source of 0.5 metric. The tests were performed in a special room of the microbiology laboratory, equipped without metal furniture and in absence of people. Suspensions of bacterial cells were spread on a solid medium in Petri dishes and then placed, without cover, under the ion generator, at different distances from the source, for 24 hours. The results show a greater effect of the negative air ions on the Gram- than on the Gram+ cells; the number of colonies of Escherichia coli grown under ionization was estimated to be 15.1 times less than of the colonies grown without ionization, while the growth of Staphylococcus aureus in the same conditions was only 4.5. time less. No contamination of Petri dishes by environmental bacteria has ever been occurred during trials: this could demonstrate a biocidal effect of the negative ionization also on air bacteria.