The sensitivity of diploid human fibroblasts to the cytotoxic effects of diphtheria toxin (DT) depended on cell growth status. Exponentially growing cells treated with 10(-3)-1 lethal flocculating units (LF) of DT/ml for 4 days survived with a frequency of 4 x 10(-4). However, the DT-resistant phenotype of colonies isolated under these conditions was not stable. When the growth of the cells had been arrested by confluence or deprivation of serum growth factors prior to treatment with DT (4 days, 10(-3)-0.6 LF/ml), the survival decreased to 2 x 10(-6) and the resistance of isolated colonies was stable. An in situ assay for induced DT-resistant mutants was developed in order to avoid problems associated with the possible reduced viability of the mutants relative to that of wild-type cells. A reproducible and linear dose response was obtained for the induction of DT-resistant mutants by ethylnitrosourea. The mutants were induced with high frequency by this compound (e.g., 10(-3) mutants/viable cell at a 37% survival dose); complete expression of the mutant phenotype occurred after 6 generations of growth under nonselective conditions. Isolated mutant colonies showed stable resistance to DT and were cross-resistant to Pseudomonas aeruginosa exotoxin A.