We estimated cholinesterase (EC 22.214.171.124) activities in erthrocytes and plasma of 443 men, 188 women not taking oral contraceptives, and 70 women who were taking oral contraceptives. Men in the first six decades of life had higher plasma cholinesterase activity than did women who were not taking oral contraceptives, and these women had higher plasma cholinesterase activity than women taking oral contraceptives. After the age of 60 there was no intersex difference. Activity of erythrocytes from the oral contraceptive group was higher than in the other groups, men had the lowest activity, and there was an increased activity with age in both sexes until the age of 60. These findings suggest that there is no single "normal" value for cholinesterase activity for adults.