Traces (0 to 100 p.p.m.) of copper, chromium, iron, lead, and silver in used lubricating oil can be determined by atomic absorption measurement of solutions of the oil in 2-methyl-4pentanone. Calibrating solutions are prepared by dissolving commercially available organo-metallic compounds in 2-methyl-4-pentanone containing the appropriate concentration of unused oil. The results agree satisfactorily with those obtained by atomic absorption or colorimetric measurement of solutions of the ashed specimens, and the technique is in routine use for the determination of these metals in the used lubricating oil from railway diesel engines.