The relationships between blood value of Pb and activity of Ala-dehydrase erythrocyte enzyme were studied in relation to the following variables: age, activity, residence, smoking habits, drinking habits. 200 adults, subdividable into occupationally exposed were examined. The data obtained showed a relationship between smoking and Aladehydrase; a relationship was also found to exist between alcohol and blood Pb and inhibition of Ala-dehydrase activity.