The genetic dissection of virulence of Corynebacterium diphtheriae is being continued. Employing a tissue culture method developed in this laboratory, seventeen phage mutants altered in the structural gene for toxin have been isolated. These fall broadly into two groups; those which give no cytotoxicity but still produce a protein which cross- reacts serologically with toxin, and those which exhibit neither cytotoxicity or serological cross-reaction with toxin. We are now studying the latter group of mutants in an effort to characterize the defect in toxin or toxin-related protein synthesis. We are also engaged in efforts to isolate two other classes of mutants. The first, conditional phage mutants which have a temperature sensitive defect in the structural gene for toxin; the second, bacterial mutants capable of producing toxin at high iron concentrations.