The effect of tetrapyrrole compounds on the biosynthesis of corrinoids and porphyrins in P.shermanii was studied. The addition of bilirubin to the medium resulted in a remarkable increase in accumulation of porphyrins in the culture broth and formation of corrinoids in the cells. The amounts of corrinoids and porphyrins formed by addition of 103μM of bilirubin were about 1.5 times as much as those of the control. δ-Aminolevulinic acid synthesis in the growing cells of P.shermanii was inhibited by hemin. Conversion of porphobilinogen to porphyrins was supposed to be regulated in the same manner. The addition of cyanocobalamin resulted in an inhibition of de novo synthesis of corrinoids. Cobalamin and coproporphyrin were the major products under the conditions tested.