Globin was prepared from the main (A0) component of human haemoglobin and reacted with protoporphyrin IX; the product, when subjected to chromatography on CM-Sephadex, separated into fast- and slow-moving species. These were isolated for examination. The dissociation constant for the tetramer–dimer equilibrium of fast-moving porphyringlobin was determined at 2.8×10−6m; this is to be compared with values of 2.2×10−6m and 8×10−8m determined for oxyhaemoglobin and the slow-moving porphyringlobin respectively. It was also shown that the thiol groups of fast-moving porphyringlobin react with 4,4′dithiodipyridine at an identical rate with those of oxyhaemoglobin; in comparison, the rates of reaction of deoxyhaemoglobin and porphyringlobin are much slower but are again identical with one another. The quenching of porphyringlobin fluorescence by I− ions was also studied. The quenching could not be represented by a simple Stern–Volmer relationship (whereas that of porphyrin–apomyoglobin is), but was represented by a model in which the fluorescence of fast-moving porphyringlobin was more accessible to the quencher than that of the slow-moving component. Similarly, fast-moving porphyringlobin was photodecomposed more rapidly by oxygen than the slow-moving species.