Diphtheria toxin kills spheroplasts of S. cerevisiae but not the intact yeast cells. After 2 h of exposure to ca 10-7M toxin, less than 1% of spheroplasts were able to regenerate into intact cells. The same high levels of toxin inhibited the rate of protein synthesis by more than 90% within 1 h, whereas RNA and DNA synthesis were not inhibited until 4 h of exposure. Both killing and protein synthesis inhibition were dependent on toxin concentration. The nature of the toxin-cell interaction was also studied by using fragments of intact toxin and mutant of toxin proteins. Neither toxin fragment A nor CRM45 nor CRM197 affected spheroplasts, but CRM197 and ATP prevented the inhibitory action of intact toxin. These results suggest that toxin acts on S. cerevisiae spheroplasts in much the same manner as it acts on sensitive mammalian cells.