Antibacterial activity of negative air ions (NAI) on Escherichia coli and Pseudomonas fluorescens has been investigated. Appropriately diluted cell suspensions of E. coli and P. fluorescens were inoculated onto agar and exposed to NAI (> 2 x 10(6) ions/cm(3)) in an airtight chamber. Although no bacterial killing was observed after short exposure (15-30 min), longer exposure upto 4 hours caused 33% and 42% reduction in viability of E. coli and P. fluorescens, respectively. Maximum killing was observed when the plates were positioned in front of the NAI generator. Young exponential phase cells of P. fluorescens were more resistant (8% reduction in viability) as compared to stationary phase cells (42% reduction in viability). Also, the starved cells displayed less reduction in viability (33%) as compared to normal cells (42%). The susceptibility of bacterial strains to NAI varies depending upon the strain type, their physiological state as well as the distance/orientation with respect to the source. Although application of NAI for decreasing the ambient microbial load is a valid option; extended exposures might be required for controlling physiologically different cells such as exponential phase cells and/or starved cells.