A system for the rapid screening of urines for the presence of bacteriuria has been devised using a Coulter Counter Model ZBI linked to a multichannel pulse-height analyser (Coulter "Channelyser") with computer analysis of the output. In a series of 215 urines containing growth of a single pathogen of more than 100 x 10(6)/l (greater than 100 000/ml) satisfactory level of sensitivity (99.1%) was obtained using only two different amplification settings by means of a brief treatment (5-10 seconds) of the undiluted specimen with low intensity ultrasound; 85-90% of mixed growths of 10 x 10(6)/l (greater than 10 000/ml) were detected. Sonication did not improve the results in this group. Specimens showing abnormal cyturia of more than 10 x 10(6)/l (greater than 10 000/ml) but no growth on culture were positive in 33% of cases without the use of ultrasound but in 72% after sonication.