A highly specific and sensitive procedure for determining histidase activity, with labeled histidine as the substrate, that requires only 1 to 2 mg of stratum corneum epidermidis has been developed. Methods for determining ratios of urocanic acid to histidine (U/H) in stratum corneum epidermidis and for measuring excretion of histidine and its metabolites by thin-layer chromatography were developed. Application of this triad--stratum corneum epidermidis histidase determination, measurement of stratum corneum U/H, and analysis of excretion of histidine metabolites--should facilitate evaluation of the condition of infants and young children with suspected histidinemia. Moreover, differentiation between patients with the classic form and genetic variants of the disorder should now be possible.