The frequency of isoniazid slow inactivators is very high among Eqyptians, 82% have more than 0.91 γ/ml of free INH in serum specimens obtained six hours after a single dose of 10 mgm/kgm. The incidence of the allele for INH acetylase enzyme deficiency is estimated at 0.9059 and that of the heterozygous carrier of this allele at 0.1705. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company