Tetanus toxin, fragment B and fragment C were assayed for toxicity on the mouse phrenic nerve-hemidiaphragm preparation. The native toxin was a potent blocker of neuromuscular transmission; fragment B possessed little toxicity and fragment C was atoxic. Pretreatment of tissues with fragment C antagonized the neuromuscular blocking properties of tetanus toxin, but not those of type A botulinum toxin or beta-bungarotoxin. Agents that antagonize cholera toxin (B subunit) and diphtheria toxin (CRM197) did not antagonize tetanus toxin and did not alter the ability of fragment C to antagonize tetanus toxin. Fragment C exerted its effect by competing with unbound toxin for receptor sites on the nerve membrane. The fragment did not: 1) displace bound toxin; 2) inhibit internalization of toxin; or 3) inhibit intracellular expression of toxicity. In assays on intact cells, under conditions in which toxin binding was not dissociable, fragment C binding to phrenic nerves had an apparent KD of approximately 1.4 X 10(-7) M. Homogenates of mouse cerebral cortex adsorbed tetanus toxin and these homogenates competed with phrenic nerves for unbound toxin. Homogenized cortex did not displace or promote desorption of toxin already bound to phrenic nerves. Homogenates of eel and torpedo electric organ were not very effective in adsorbing toxin.