A total of 1,002 urine specimens were evaluated by laser nephelometry. This technique was compared with both colony counts, done with a calibrated loop, and serial dilutions. For urine specimens containing between 104 and 105 bacteria per ml, laser nephelometry detected 75.4% of those detected by colony coùnt and 65.6% of those detected by serial dilution. For specimens where the concentration of bacteria was greater than 105 per ml, laser nephelometry detected 95.8 and 92.4% of those detected by colony count and serial dilution, respectively. The mean detection time for bacteriuria varied from 1.57 h for more thah 105 bacteria per ml to 4.47 h for more than 104 bacteria per ml. To determine the number of bacteria according to the voltage growth curve, the passage time at 3 V was used as an index. The mean passage time at 3 V decreased from 5.18 h for fewer than 104 bacteria per ml to 1.42 h for more than 106 bacteria per ml. The mean passage time at 3 V differed significantly for different concentrations of bacteria. Thus, this index allowed us to predict the number of bacteria in the urine specimens. Laser nephelometry has been used for many years for the immunological determination of proteins; it can now also be considered a tool for rapid screening in bacteriology.