The highly toxinogenic Park-Williams 8 strain of Corynebacterium diphtheriae grows slowly in vitro and is avirulent. C. diphtheriae Park-Williams 8 is defective in iron uptake and does not produce the corynebacterial siderophore corynebactin. Addition of partially purified corynebactin' stimulated iron uptake and growth of iron-deprived C. diphtheriae Park-Williams 8 cells. Corynebacterium diphtheriae Park-Williams 8 (PW8), isolated in 1896 by Park and Williams (13), is used as the source of toxin for the production of diphtheria toxoid for immunization. The PW8 strain is atypical; it produces unusually large amounts of diphtheria toxin in vitro, and it grows so poorly in the tissues of experimental animals after intracutaneous inoculation that it is considered to be avirulent (3). The ability to acquire iron in environments where iron is tightly bound to host iron-binding proteins is an essential attribute of virulence for many pathogenic bacteria (8, 17). Growth of C. diphtheriae is dependent on an adequate supply of iron, and production of both diphtheria toxin and coproporphyrin III by C. diphtheriae is regulated by iron (reviewed in reference 1). The metabolism of iron has an important role in regulation of toxinogenesis in all strains of C. diphtheriae studied so far and may also be an important determinant of virulence in this organism. Other factors considered to be important for virulence in C. diphtheriae have been discussed elsewhere (1-3, 10, 12). Recent studies in our laboratory have provided preliminary biochemical and genetic information about iron transport in C. diphtheriae. Iron uptake into C7 strains occurs by an active transport system with a high affinity for the ferric ion (16). Several mutants of strain C7(,B) selected for their ability to produce diphtheria toxin during growth in high-iron medium have severe defects in iron uptake (6). Such mutants grow slowly under low-iron conditions and exhibit an irondeprived phenotype even in high-iron medium. During growth under iron-limiting conditions, C7 strains of C. diphtheriae produce a siderophore (corynebactin) which is necessary for high-affinity ferric iron uptake (15). We postulated that corynebactin was a novel siderophore because it did not give positive tests for either o-dihydric phenolate groups or hydroxamate groups, and it was not utilized by any of a variety of other bacteria tested. A mutant of C7(p),
