Ortho-phthalaldehyde (OPA) received clearance by FDA in October 1999. OPA solution is a clear, pale-blue liquid (pH, 7.5), which typically contains 0.55% OPA. OPA has demonstrated excellent microbiocidal activity in in vitro studies[2,3]. For example, it has shown superior mycobactericidal activity (5-log10 reduction in 5 minutes) compared with glutaraldehyde. The mean time required to effect a 6-log10 reduction for M. bovis using 0.21% OPA was 6 minutes, compared with 32 minutes using 1.5% glutaraldehyde (Table 2)[4]. When tested against a wide range of microorganisms, including glutaraldehyde-resistant mycobacteria and Bacillus subtilis spores[5], OPA showed good activity against the mycobacteria tested, including the glutaraldehyde-resistant strains, but 0.5% OPA was not sporicidal within 270 minutes of exposure. Increasing the pH from its unadjusted level (about 6.5) to pH 8 improved sporicidal activity.