In a series ofin vivo andin vitro experiments employing crude and purified tetanus toxin labelled with fluorescent dyes, the following observations were made: a) Tetanus toxin is bound to muscle and brain, but not to liver, cardiac muscle, spleen, kidney or lung. b) Since these tissues will not bind serum albumin or tetanus anti-toxin or rhodamine to the same degree, even when these are present at twenty times the concentration of the toxin, this binding indicates tissue selectivity towards tetanus toxin. c) Mitochondria isolated from brain were stained by rhodamine-labelled tetanus toxin, but liver mitochondria were not stained under the same experimental conditions. d) Treatment of the test animals with either antitoxin or toxin before sacrifice reduced the apparent amount of toxin bound to musclein vitro but had little effect on the ability of brain to bind the toxin. e) After toxin was bound to both muscle and brainin vitro, valences were still available for the binding of antitoxin. Since the amount of antitoxin bound to muscle or brain was greatly increased by prior incubation of the sections with toxin it follows that this is an example of specific immunological binding. Tetanus toxin is bound to muscle and brain, but not to liver, cardiac muscle, spleen, kidney or lung. Since these tissues will not bind serum albumin or tetanus anti-toxin or rhodamine to the same degree, even when these are present at twenty times the concentration of the toxin, this binding indicates tissue selectivity towards tetanus toxin. Mitochondria isolated from brain were stained by rhodamine-labelled tetanus toxin, but liver mitochondria were not stained under the same experimental conditions. Treatment of the test animals with either antitoxin or toxin before sacrifice reduced the apparent amount of toxin bound to musclein vitro but had little effect on the ability of brain to bind the toxin. After toxin was bound to both muscle and brainin vitro, valences were still available for the binding of antitoxin. Since the amount of antitoxin bound to muscle or brain was greatly increased by prior incubation of the sections with toxin it follows that this is an example of specific immunological binding. The significance of toxin binding by tissue sections and isolated mitochondria relative to the mode of action of tetanus toxin is discussed.