We have developed a system to study the interactions of diphtheria toxin with the cell surface using non-toxic mutant proteins synthesized in vitro. Proteins obtained by N-terminal deletions containing the whole B fragment bound strongly to cells. Deletions extending into the B fragment did not yield an autonomous binding domain. Loss of only the N-terminal 3 kd of the B fragment significantly impaired the ability to recognize the receptor. This, together with previous reports that the C-terminal end of the B fragment is required for binding, suggests that both ends of the B fragment are necessary for receptor recognition. Receptor bound diphtheria toxin undergoes a conformational change at pH less than 5.3 that results in translocation of the A fragment to the cytosol and the appearance of a B fragment-derived 25 kd polypeptide (P25) resistant to externally applied protease. Only the B fragment was required for generation of P25. N-terminal deletions of 130 amino acids or more resulted in proteins that gave rise to P25 at higher pH than full length toxin. Furthermore, a second protease-inaccessible polypeptide of 18 kd (P18) was observed.