The basic polyamine spermine was tested for antibacterial activity at two pH levels by the modified cup method against a variety of gram-positive and gram-negative organisms isolated from urine. At pH 6.4, with concentrations ranging from 39 to 2,500 μg per 0.1 ml, there were no clear zones of inhibition seen with any of the gram-negative test organisms, although some adverse effect on growth within the area of the cylinder was noted in 36%. Three of 17 gram-positive strains were inhibited at this pH. Spermine was more active at pH 7.4, but even at the highest concentrations only 16% of the gram-negative and 47% of the gram-positive bacteria tested showed definite zones of inhibition. It is concluded that spermine probably plays little, if any, role in natural resistance to urinary tract infections in vivo. Images