Expression of the rate of inactivation of isonicotinoyl hydrazide (INH) as a first-order constant gave better discrimination of slow and rapid inactivators than did the level of INH at the end of 6 hr. The most probable division point between the two classes was at k = 0.130. Rate of inactivation by obligatory heterozygotes was determined. Such heterozygotes could not be adequately differentiated from homozygous rapid inactivators. The gene frequency for slow inactivation was 0.46 +/-0.04 in Alaskan Eskimos and 0.62+/-0.06 in Athabaskan Indians.