Bis(pyridinium quaternary salts), typically those of the formula: ##SPC1##wherein R.sup.1 and R.sup.2, which may be the same or different, are each an alkyl having 8 to 14 carbon atoms, n-undec-10-yl, 2-n-hexyloxyethyl, 2-(2-butoxyethoxy)ethyl, 3,4-dichlorophenyl, 4-chlorobenzyl, 2,4-dichlorobenzyl or 2-naphthylmethyl; A.sup.1 and A.sup.2 are each a direct linkage or a linking group of the formula:--CH.sub.2 CO.NH--wherein the methylene is joined to the pyridine nitrogen; (X.sup.1.X.sup.2).sup.2.sup.- represents two mono-anions or a dianion selected from the group consisting of two halide, methanesulphonate, toluene-p-sulphonate or acetate anions or the sulphate dianion; and A.sup.3 is a linking group of the formula:--(CH.sub.2).sub.n H.CO.NH(CH.sub.2).sub.y NH.CO.NH(CH.sub.2).sub.n --Wherein n is 0, 1 or 2; and y is 0 or 2 to 6. A process for making these salts, compositions containing them and methods of preventing the growth of, or killing, bacteria by applying one of these salts to a bacterially infected environment, are also disclosed.