The binding of formaldehyde with tetanus toxin under mild detoxificatIOn conditions (0 �1M phosphate buffer, pH 7� 0, O� 005-0� 06M HCHO, 20-37�C) has been studied quantitatively by using [14C]formaldehyde. When detoxification was followed with time (0� 005M HCHO, 37�C) the toxin showed a characteristic immediate drop in lethality (~80%), followed by a slower detoxification, whereas the combination of the HCHO was a more gradual process. When a fixed time period was used (5 days, 20 and 37�C) at least O' 02M HCHO was required to destroy the lethality of the toxin towards mice, and at this concentration approximately 60 moles of HCHO were bound per mole of toxin. Fractionation of a partially detoxified toxin indicated the presence of a range of molecular species with varying formaldehyde binding and lethality.