A review is presented of the literature regarding the nature, site of production, physiological function and pharmacological function of serum cholinesterase. A study of the literature indicates that the following conditions may be associated with deficiency of serum cholinesterase: liver disease; chronic anaemias; malignant states; cardiac failure; severe acute infections; malnutrition; tuberculosis; uraemia; collagen diseases; surgical shock; severe burns; blood dyscrasias; exposure to organophosphorus compounds; haemodialysis; therapeutic radiation; treatment with albumin, AB-132 and certain phenothiazine drugs. The literature relating to serum cholinesterase as a liver function test is reviewed, with reference to the changes in liver function after surgery, and in the assessment of chronic liver disease and venous shunt operations. The genetic aspects of serum cholinesterase deficiency are briefly reviewed.