Five blue-green and one red algal species produced carbon monoxide during photosynthetic growth. The blue-green algae synthesized CO and phycocyanobilin in equimolar quantities at identical rates. The red alga, Porphyridium cruentum, incorporated z-aminolevulinic acid-5-14C into phycoerythrobilin and CO. The ratio of the specific radioactivity of phycoerythrobilin to that of CO, and the kinetics and stoichiometry of phycocyanobilin and CO formation suggest that linear tetrapyrroles in plants are derived by the porphyrin pathway via the intermediate formation of heme. The similarity between bile pigment production in algae and in mammalian systems is discussed.Phycocyanobilin and phycoerythrobilin are linear tetrapyrroles covalently linked to apoproteins occurring in the photosynthetic apparatus of red, blue-green, and cryptomonad algae (17). These compounds are structurally related to bilirubin, the principal mammalian bile pigment derived from hemoglobin in senescent erythrocytes (1,14, 18,19). It is known that CO is produced endogenously in normal man (24), and that approximately 1 mole of CO is produced per mole of heme catabolized (2, 1 1). CO formation has been described in algae (6,12,15), in fungi (23, 33), and in higher plants (22,34). CO produced in the fungus, Aspergillus niger, has been shown to arise from the breakdown of rutin to phloroglucinol and protocatechuic acid (23). Nevertheless, in most reports concerning algae and higher plants, the metabolic origin of CO has not been established. We have shown that in the green algae, Cyanidium caldarium, CO is a by-product of phycocyanobilin formation (31). The present paper describes some quantitative and kinetic parameters of CO and bile pigment formation in five species of blue-green algae and in the red alga, Porphyridium cruentum.