“…First, there is the uncertainty regarding the degree of reabsorption of urobilinogen from the bowel. Second, it has been shown from studies with nitrogen -15 labelled glycine that a significant proportion of urobilinogen, probably in the region of 15% in normal individuals, and more in haemolytic disorders, is derived from sources other than haemoglobin within mature circulating erythrocytes (London, West, Shemin and Pittenberg 1950); a number of porphyrins of endogenous origin, such as myoglobin, probably contribute to the total urobilinogen (Lemberg andLegge 1949, Schmid 1959); it is also suggested (Bingold 1949) that there may be oxidation products of haem, such as dipyrrholes, which are not included in the measurement of urobilinogen. These different findings undoubtedly help to explain why a number of investigator (Hagen andMacDonald 1954, Crosby 1955, Baldini and di Pietrantoni 1957) have failed to recover the expected amount of urobilinogen from patients with various haemolytic disorders.…”