Summary.Low concentrations (0.025–0.125%) of glutaraldehyde inhibited or prevented colony formation byEscherichia coli, Bacillus subtilisandB. pumilisin agar, and inhibited germination of spores of theBacillusspp. in L‐alanine plus D‐glucose. Higher concentrations (2%) of glutaraldehyde at pH 8.5 were sporicidal. Pre‐treatment of spores with glutaraldehyde lessened release of dipicolinic acid when the spores were subsequently heated at 100°, but not at 121°. Spores treated with glutaraldehyde and then with 0.5 M thioglycollic acid in 6 M urea at 70° were less sensitive to lysis by hydrogen peroxide than spores which had not been exposed to glutaraldehyde. Glutaraldehyde was less effective in preventing peroxide induced lysis if added to spores which had been previously exposed to thioglycollic acid plus urea at 70°. The mechanism of the sporicidal activity of glutaraldehyde is discussed in relation to the