Abstract: Novel solid antibacterial compositions suitable for use in an oil or gas well comprise at least 50% by weight of a solid antibacterial nitroalkanol compound active against sulphate-reducing bacteria and a carrier, the composition having a density of at least 1.2 g/cm.sup.3 and being adapted to dissolve, disintegrate or disperse in water but not in oil. A preferred antimicrobial compound is 2-bromo-2-nitropropane-1,3-diol. The antibacterial compositions are introduced into the aqueous layer in the sump at the bottom of the collecting system of the oil or gas well. This inhibits corrosion in the metal work of the oil or gas well, for example the pipe lines of the collecting system.