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Phycoerythrobilin, one of two tetrapyrrolic prosthetic groups in R-phycoerythrin, was the source of two crystalline urobilinoid pigments. One (catalytic urobilin peb) was prepared from phycoerythrobilin by hydrogenation followed by partial reoxidation of the resulting chromogen, and the other (isomeric urobilin peb) by prototropic isomerisation of phycoerythrobilin while still attached to protein. Both resembled d-urobilin in possessing high dextrorotatory optical activity in chloroform solution and it is concluded that phycoerythrobilin contains one assymetric centre which has the same absolute configuration as the two centres shown previously in d-urobilin. Isomeric urobilin peb must be formed under the influence of a stereospecific­directing influence which leads to its containing two assymetric centres having the same absolute configuration as those in d-urobilin. The optical activity of both urobilins from phycoerythrobilin was greatly reduced by methanol and trifluoroacetic acid, suggesting that in non-polar solvents they are internally hydrogen bonded. [ABSTRACT FROM AUTHOR]