Monoclonal antibodies against fragment A of diphtheria toxin were isolated and characterized. Three antibodies with similar affinities for fragment A had different effects on the NAD: EF2-ADP ribose transferase activity of fragment A; i.e., antibody DA1 almost completely inhibited the enzymic activity at a molar ratio of one, whereas DA2 inhibited only partially and DA3 had no effect. However, when fragment A176 from the mutant toxin CRM176 (about 1/10 as active as wild type) was used, DA2 proved a more effective inhibitor than DA1. The affinities of these antibodies for the enzymically inactive mutant fragments, A197 and A228, were significantly less manifest than for wild-type fragment A. Binding of the antibodies to whole toxin and the chain termination mutant CRM45 was weak. When DA2 was introduced into Vero cells growing in monolayers, by using the red cell ghost fusion method, the cells became resistant to CRM176. The anti-fragment A antibodies may serve as the basis of a simple method for selection of cells into which other molecules have been co-introduced.