Voided, midstream urine specimens from 127 patients and volunteers with sterile bladder urine were incubated for 24 hours to measure the presence or absence of inhibitory activity in urine to small numbers (100 or less bacteria per milliliter) of contaminating Enterobacteriaceae (Escherichia coli, Proteus, Klebsiella), enterococci and several species of the normal perineal flora. Virtually no inhibitory activity was found for Enterobacteriaceae, enterococci or Staphylococcus epidermidis when measured by substantial growth at 24 hours. Of the 32 patients with 100 or less Enterobacteriaceae per milliliter urine before incubation, 30 urine specimens had growth of 10(5) bacteria per milliliter or more of the same organism; 9 of 30 specimens began growth at less than 10 Enterobacteriaceae per milliliter urine. The 39 urine specimens containing 10(4) or more enterococci or Staphylococcus epidermidis after 24 hours of incubation began with original inocula of 100 bacteria per milliliter or less; 28 of 39 began growth at less than 10 bacteria per milliliter. Growth rates of pure strains inoculated into sterile, filtered urine (strains derived from from the same individual as the urine in which they were inoculated) showed significant differences in bacterial growth at 4 to 6 hours after incuation; Escherichia coli and enterococci generated 10 to 100 times more colonies than strains from the normal perineal flora (lactobacilli, corynebacteria and so forth). We conclude that normal urine is non-inhibitory to small inocula of Enterobacteriacea and enterococci but that it is not as conducive to growth for some of the indigent, resident flora, perhaps explaining the relative infrequency of urinary infections owing to normal, perineal bacteria.