Powders having contact biocidal properties comprise a polysaccharide carrying atomic/metallic silver. The preferred polysaccharide is chitin, although other polysaccharides including chitosan, carboxymethyl celluloses and carrageenans can be used. The chitin may be obtained from deproteinated crustacean shells without demineralisation, thus being admixed with calcium carbonate and other naturally occurring minerals present in the shells, and may be enzyme deacetylated. The powders of the invention can be used as biocidal dusting powders, formulated into pastes, gels, hydrogels, creams, foams and aerosol sprays for pharmaceutical applications, or dissolved to form solutions for coating substrates such as skin, fabrics, glass, leather and paper to give a bactericidal surface. A particular application of such a solution is as a protective, post-wash treatment for workwear in a laundering process. The powders of the invention may be prepared by slurrying a polysaccharide, which is capable of interacting with silver ions and which is in powder form, in a liquid in which the polymer is insoluble, which liquid contains silver ions, filtering off the powder, washing the powder, reducing the silver ions which have interacted with the polysaccharide to atomic/metallic silver, and drying the powder. According to the polysaccharide chosen, the liquid is suitably water or aqueous ethanol. The silver ions may derive from silver nitrate. The reduction of the silver ions which have interacted with the polysaccharide to atomic/metallic silver can be effected photochemically through exposure to light. To hasten the reduction, however, the washed powder is preferably slurried in a solution of an alkali metal halide, irradiated under stirring with natural or artificial light containing an ultraviolet component, and again filtered off and washed, before drying it.