Age-associated changes in the microsomal mono-oxygenase enzyme, aryl hydrocarbon hydroxylase (AHH), were investigated in peripheral blood monocytes isolated from young and elderly subjects. Young subjects were selected from the age group < 35 years; elderly subjects were > 70 years. AHH activity, expressed as nM 3-hydroxy benzopyrene formed/10⁶ cells/h of incubation, was similar in young and elderly non-smokers (elderly = 6.7 ± 1.8; young = 9.1 ± 0.9; p > 0.05). In smokers, contrary to some previous reports, AHH activity was not significantly higher than in non-smokers of similar age (elderly smokers = 8.6± 1.0; young smokers = 10.5 ± 2.4; p > 0.05). There was no significant difference between young and elderly smokers (p > 0.05).We conclude that: (a) AHH activity is similar in young and old non-smoking individuals; (b) no significant differences were detected in in vivo induction of this enzyme activity between smokers and non-smokers.