A procedure has been developed for the assay of urinary porphyrins. Total porphyrins are extracted from acidified urine into n-butanol. The fluorescence of this solution under ultraviolet illumination gives a preliminary indication of abnormal levels of porphyrins. They are then separated and identified by thin-layer chromatography. Uroporphyrin and coproporphyrin can be qualitatively estimated by visual inspection or accurately quantitated. For quantitation, the section of silica gel containing each porphyrin is scraped from the plate, suspended in glycerol solution, and the fluorescence measured directly without elution of the porphyrins from the silica gel. This assay, which yields either qualitative or quantitative information, is suitable as a diagnostic procedure in the clinical laboratory.