A chromatographically homogenous preparation of the binding fragment from tetanus toxin was tested for its ability to antagonize the neuromuscular blocking properties of native tetanus toxin. At a concentration of 1 X 10(-6) M, the binding fragment antagonized the paralytic effects of native toxin (1 X 10(-9) M) on mouse phrenic nerve-hemidiaphragms. The binding fragment of tetanus toxin (1 X 10(-6) M) also was tested for its ability to antagonize types A to G botulinum neurotoxin. The fragment did not produce statistically significant antagonism of types A, B, D, F and G neurotoxins, but it did produce highly significant antagonism of types C and E neurotoxin. A series of experiments involving column chromatography, dialysis and high-performance liquid chromatography confirmed that the binding fragment rather than a contaminant was responsible for antagonism. Experiments with type C neurotoxin showed that antagonism between the binding fragment and the toxin occurred extracellularly at the level of the cell membrane. The fragment did not act directly on the toxin to produce inactivation, but instead competed with the toxin for a binding site on the membrane. The fact that the binding moiety of tetanus toxin and the binding moiety of botulinum toxin compete for a similar membrane site suggests that the molecules have areas of structural homology. Such homologies, if confirmed, would have important therapeutic implications.