SUMMARY: The fixation of tetanus toxin by nervous tissue (Wassermann-Takaki phenomenon, 1898) has been re-investigated. Fixation is greatly dependent on the concentration of toxin and of receptor. Toxin is fixed in preference to toxoid, but its toxicity does not appear to be markedly diminished by fixation. A biological assay of the toxin receptor in nervous tissue and its components has been devised. The receptor activity is not due to a cerebroside, as thought by Landsteiner & Botteri (1906), but to some substance (present mainly in grey matter) that tends to associate with it.