IN 1961 the urines of two children with speech defects were found, by Ghadimi et al to contain a substance that reacted with ferric chloride or reagent strips (Phenistix)* to produce a green color1; increased levels of histidine were measured in their blood and urine. Investigation of these and other children with this condition, since named histidinemia, established that it results from a metabolic error, specifically, defective activity of the enzyme histidine α-deaminase. The immediate consequence of this enzymatic deficiency is to reduce or block formation of urocanic acid from histidine with resultant accumulation of histidine in body fluids. Because of histidine accumulation, a secondary pathway for histidine degradation is activated and the production of imidazolepyruvic acid and imidazolelactic acid are increased,2,3 and they are excreted in the urine. It is imidazolepyruvic acid that reacts with FeCl3 and reagent strips.*3,4 Histidine αdeaminase activity has been found