Maleic, fumaric, dibromosuccinic, succinic, citraconic, mesaconic and itaconic acids were studied. The trans acids were superior to their cis isomerides in (a) bactericidal action, (b) as protein precipitants, and (c) in their effect upon enzymatic reactions. The cis acids are more strongly adsorbed by proteins. The uptake of both forms of acids by proteins is probably an adsorption reaction; no evidence for chemical combination with the protein was obtained. Fumaric acid is a stronger disinfectant than is phenol, but is much less toxic to higher animals. It is suggested that trans derivatives may prove to be useful germicides.