An in vitro method is provided that is suitable for determining the potency of diphtheria antitoxin (antibodies against the diphtheria toxin) in preparations of plasma-derived human immune globulins. Diphtheria toxin is produced by Corynebacterium diphtheriae and has the ability to produce a cytopathogenic effect on susceptible epithelial cell lines. The test is based on the ability of diphtheria antitoxin to neutralize the diphtheria toxin, decreasing its cytotoxic effect. Specifically, the test determines the potency of the diphtheria antitoxin based on its ability to inhibit the cytotoxic effect of diphtheria toxin on cultured Vero cells (African green monkey kidney epithelial cells) relative to a reference standard. The mitochondrial dehydrogenases of live Vero cells can reduce the dye 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) to a blue–black product that is then measured by absorbance at 540 nm. If little or no diphtheria antitoxin is present, then diphtheria toxin induces cell death and the inability of cells to reduce MTT, resulting in the presence of white or colorless wells. Acceptance criteria are defined by the appropriate regulatory agencies.