Lead is assayed in urine by atomic absorption spectrophotometry after separation by coprecipitation with a CaCO3, HCl, (NH4)2HPO4 solution at pH 9. Concentration of lead and removal of interfering substances are achieved simultaneously. Fresh or old urine samples acidified with HNO3 and thymol can be used. Precipitation is quantitative. The precision of the method is ±0,004 ppm of lead.