Summary.Glutaraldehyde in 2% aqueous alkaline solution is rapidly bactericidal and sporicidal, killing 99.99% of the spores ofBacillus anthracisandClostridium tetaniin 15 and 30 min, respectively. It exhibits some degree of tuberculocidal activity, but it appears to be inferior to several other disinfectants when challenged with a large inoculum. Fungicidal action has been demonstrated but not assessed quantitatively. Evidence is presented to show that biocidal activity depends on the availability of two free aldehyde groups in the molecule.