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A recent review (1) has dealt with previous studies of the urinary coproporphyrins, both as carried out in this laboratory and elsewhere. The purpose of the present series of communications is to provide newer data relating to various aspects of the coproporphyrin problem.It is well known that two of the four coproporphyrin isomers occur in human urine (1).. These naturally occurring isomers are types I and III according to H. Fischer's classification, as based upon the configuration of the four artificial aetioporphyrins (2). Apart from the difference displayed by the two isomers in ester melting point and crystal habitus, a differing physical behavior has been described Us manifested in their pH fluorescence curves. This difference was first noted by Hoerburger and Fink (3-5) who attempted to apply it to the coproporphyrin of normal urine. The crystal habitus and pH fluorescence curve of the few crystals which they obtained indicated the presence of coproporphyrin I (5). A later study by Watson (6) was likewise somewhat indecisive in that the melting point of the crystalline ester was 2280 C, that of pure coproporphyrin I methyl ester being 2520 C (corrected), as contrasted with the varying or dimorphic melting point of the esters of coproporphyrin III: 1340, 1420, 1680 (2). The crystal habitus in this study also, was that of the type I isomer, and it was therefore concluded that in the urine employed, which was from one normal individual, the preponderant coproporphyrin was type I. At the same time it was believed that a small amount of the type III isomer was present, since a few rosettes of prisms having the type III habitus were obtained from the mother liquor. The weighed yield of crystals in 1 Aided by grants from the John and Mary R. Markle Foundation, New York City, and from the Medical Research Fund of the Graduate School, University of Minnesota, -Minneapolis. this study was 0.13 mgm., obtained from 33 liters of urine. This had seemed a fair sized undertaking, but it was dwarfed by the subsequent investigation of Grotepass (7), in which 10,000 liters of presumably normal human urine were worked up with a yield of ca. 200 mgm. of crystalline coproporphyrin ester. Grotepass' method was to adsorb the porphyrin on infusorial earth, large containers being employed into which many individuals passed their urine, the urine simply being decanted after a period of contact with the earth. The porphyrin which was isolated after subsequent purification and fractional crystallization was, found to consist of about equal parts of type I and III isomers, and it was therefore concluded that the normal urinary coproporphyrin consists of approximately equal parts of the two types. Certain reservations regarding this conclusion will be discussed subsequently.During the past nine years studies have been in progress which were designed to provide a method of isomer analysis applicable to a small volume of urine and by means of which it would be possible to make a systematic study of the urinary coproporphyrin isomer excr...