A high-speed reversed-phase high-performance liquid chromatographic method using an octadecylsilyl 3 cm long (3 microns particle size) column to separate the free acids of uroporphyrins I and III and coproporphyrins I and III from each other, and from the type I isomers of several other porphyrin carboxylic acids, is described. Separation of the porphyrins was achieved in less than 8 min, and injections were possible every 12 min. The detection limits of uroporphyrin, coproporphyrin, and mesoporphyrin were 75, 45, and 35 fmol (at a signal-to-noise ratio of 2), respectively. Application of the method to the determination of urinary and liver porphyrin patterns is shown.