The mycobactericidal activity of various dialdehydes has been assessed by a quantitative suspension test in both 'clean' and 'dirty' conditions. Test organisms consisted of glutaraldehyde (GTA)-sensitive strains of Mycobacterium chelonae NCTC 946, M. abscessus NCTC 10882, two GTA-resistant M. chelonae strains and M. terrae NCTC 10856 (a proposed M. tuberculosis surrogate). The aldehydes tested were a new high-level disinfectant, ortho-phthalaldehyde (OPA) at 0.5% (v/v) unadjusted pH 6.5 and pH 8, GTA at 0.5% (v/v) pH 8, glyoxal at 0.5% (v/v) pH 8 and 10% (v/v) unadjusted pH 2.8, malonaldehyde sodium salt (NaMDA) at 0.5% (w/v) pH 8 and 10% (w/v) unadjusted pH 7.5 and succinaldehyde at 0.5% (v/v) pH 8. Results showed that 0.5% acidic and alkaline OPA were rapidly mycobactericidal, under both 'clean' and 'dirty' conditions, and more importantly were active against GTA-resistant strains. The washer disinfector isolates of M. chelonae were, as expected, extremely resistant to 0.5% GTA which was slowly mycobactericidal against the other strains. Glyoxal, NaMDA and succinaldehyde were ineffective against all the strains investigated. However, a high concentration of glyoxal exhibited a slow mycobactericidal activity except with M. terrae NCTC 10856, but this was not observed with NaMDA. This evaluation, using a quantitative suspension test based on a European standard, supported the claim that OPA is an effective choice as a high-level disinfectant for medical devices.