The enzyme from guinea-pig liver that degrades pyridine-haemochromogen into formylbiliverdin, a possible precursor of biliverdin, breaks the porphyrin ring exclusively at the alpha-methene bridge. The enzyme is therefore appropriately named haem alpha-methenyl oxygenase. 2. It is deduced that this enzyme plays an essential role in haemoglobin catabolism in vivo.