The carboxyl-terminal half of the heavy chain of tetanus toxin (Hc) contains the domain required for binding to purified gangliosides and neuronal cells. The structural requirements for the interaction of Hc with receptor were studied by generating mutants of Hc with deletions at either the carboxyl or amino terminus and characterizing their binding. A deletion of 10 or more amino acids from the carboxyl terminus resulted in a major loss of Hc binding to purified gangliosides and spinal cord neuronal cells, whereas a deletion of the carboxyl-terminal 5 amino acids did not affect binding. The removal of up to 263 amino acids from the amino terminus did not inhibit binding. Each of the truncated proteins was much more sensitive to trypsin than was full-length Hc, suggesting an alteration in conformation. The receptor binding activity of Hc was not retained in a peptide corresponding to the carboxyl-terminal 20 amino acids. These data suggest that the carboxyl-terminal region of Hc is important for maintaining a conformation necessary for binding to receptor.