A method for detecting the presence of bacteria in urine based on measuring a change in potential between two electrodes was tested in a clinical microbiology laboratory. Initial tests were conducted with 13 bacteria commonly associated with urinary-tract infections; all of the test organisms were detected within 2--9 hours. A linear relationship was established between inoculum size and the time an increase in voltage was observed on a strip-chart recorder. No response was seen with sterile urine, but urine samples inoculated with Escherichia produced the expected positive response. One hundred twenty-eight urine specimens from hospitalized persons were simultaneously tested by the electrochemical detection method (ECDM) and by conventional bacteriologic procedures. Ninety-four per cent of 49 positive samples with counts of 10(5) organisms per ml. or more were detected within 4 hours and 100% at 10 hours with the ECDM. Twenty-nine specimens with counts less than 10(5) cells per ml. were detected in 3.5 to 9 hours; two samples (8%) in this group were positive within 4 hours. Fifty samples were negative for bacterial growth, and no increase in voltage was found with these samples.