The cytotoxicity (TD) level of diphtheria toxin for human Chang liver strain was 0.1 guinea pig MLD per ml; for mouse liver NCTC 1469 strain, the TD was 500 MLD per ml. The results of cell culture toxicity correlated well with relative susceptibility of both man and mouse. The initial effects of toxin on the susceptible Chang liver cells were studied at one half the TD level (0.05 MLD per ml). At this low concentration of toxin, the number of cells per culture was reduced slightly below the "0" hr values, whereas the amounts of cell protein, RNA, and DNA were increased. Analysis of the toxin-treated cells indicated an enlargement of the cells. The individual cells contained significantly more protein, RNA, and DNA than the control cells and the cell volume was increased 1.9 times. When purified diphtheria toxin was incubated with the susceptible Chang liver cells and then tested for its biological activity, the results showed an increased diffusion rate in parabiotic culture chambers and definite cytotoxicity to the normally resistant mouse liver cells. The cytotoxicity was neutralizable with antitoxin. The results suggest that the toxin-susceptible cells transform the toxin molecule to a more active derivative which affects the highly resistant mouse liver cell.