The toxicity of many classes of compounds in snails, and probably in many other aquatic animals, is due to the ability of these compounds to inhibit oxygen uptake by interaction with enzymes which require intact-SH (i.e. Sulfhydryl) groups for normal function. Among the classes of compounds highly toxic to snails which are potent sulfhydryl inhibitors are B-nitrostyrenes, Vinyl ketones, vinyl sulfones, zinc dithiocarbamates, and quaternary salts. The sulfhydryl reactivity of representative B-nitrostyrenes and vinyl sulfones was determined by observing the change produced by the color-forming reaction between cysteine and 5, 5(1)-dithiobis (2-nitrobenzoic acid) in the presence and in the absence of the sulfhydryl reactive agent. (Author)