The ability to maintain cytochrome P‐450 enzyme levels in stored tissue samples is necessary in order to properly utilize this enzyme system in toxicological and environmental health studies. The stability of enzyme activities associated with microsomal cytochrome P‐450 was investigated to determine the best way to handle large numbers of samples and postmortem or biopsy lung and liver tissue. Long‐term storage in cold buffer resulted in diminished activities in purified microsomes and whole tissue. Benzo[a]pyrene hydroxylase and 7‐ethoxycoumarin deethylase, however, were stable when the purified microsomal fraction, postmitochondrial fraction, or whole tissue was stored at —80°C, but care should be exercised in the freezing and thawing procedures.