1. About 0.1mug.atom of combined nitrogen, in lipids and a few other compounds, can be determined quantitatively by the gentle digestion of dry samples in 10ml. test tubes with perchloric acid for 30min., followed by the estimation of the resulting ammonia as the stable blue colour (lambda(max.) 635mmu) produced by the addition of phenol and nitroprusside, an alkaline phosphate buffer and alkaline hypochlorite. 2. Deionized distilled water is required, but the other reagents need no special purification if chosen and handled with care. 3. The results are linear with from 0.015 to 0.15mug.atom of nitrogen, or up to 1mug.atom if the final solutions are diluted with water after full colour development.