SUMMARY: Studies on the kinetics of diphtheria toxin formation in iron-free culture media by variants of the PW no. 8 strain of Corynebacterium diphtheriae labelled with 14C-phenylalanine or 35S-methionine, showed that the toxin protein was synthesized de novo from amino acids by growing organisms. Release of toxin into the extracellular medium occurred without lysis of more than a minor proportion of the bacterial population.