Mutants of the human KB carcinoma cell line resistant to a cytotoxic conjugate of epidermal growth factor (EGF) and Pseudomonas exotoxin (PE) were selected. EGF-PE and the drug verapamil, which enhanced EGF-PE cytotoxicity, were used in the selection process. These mutants also showed some cross-resistance to PE. All of the EGF-PE resistant variants displayed lower levels of 125I-EGF binding, 20-50% of parental KB levels, without altered affinity for EGF and grew at a slower rate than the parental cell line KB-3-1. These results indicate that EGF-PE resistant KB cells have a complex phenotype which includes a reduction in the number of EGF receptors and reduced sensitivity to unconjugated PE. Resistance to toxin-conjugates, although pleiotropic, is specific and does not lead to resistance to multiple other anticancer drugs, nor are independently selected multidrug resistant KB lines resistant to PE. These results argue that protocols for cancer treatment could effectively use specifically designed cytotoxic toxin conjugates as an adjunct to conventional chemotherapy.