Aryl hydrocarbon hydroxylase (AHH) activity and inducibility were studied in the lymphoblasts of 47 asbestos workers and 31 control subjects. Mean inducibility (3-methylcholanthrene [MC]-or dibenz(a,h,)anthracene [DBA]-induced divided by basal enzyme activity) was greater in the occupationally exposed subjects than in the control subjects. The estimated estimated change in mean value (+/- SE) of MC inducibility was 0.88 +/- 0.21 (P less than 0.001), and that of DBA inducibility was 1.59 +/- 0.56 (P less than 0.01). Differences in inducibility values were particularly apparent in 12 pairs of spouses, in all of whom the inducibility values of the occupationally exposed subjects exceeded those of their spouses. The differences in inducibility of AHH resulted from increases in induced activity, not from any appreciable change in basal activities. They were independent of subject age, sex, smoking status, year of first exposure to asbestos, yield of lymphoblasts, or the rate of blastogenesis as measured by the incorporation of tritiated thymidine into nucleic acid. Increased AHH inducibility may bear some relation to the greatly increased risk of bronchogenic carcinoma in asbestos workers who smoke cigarettes.