The measurement of chemicals and their metabolites in the blood for biological monitoring of exposure was discussed. As the blood is the transport vehicle for chemicals and their metabolites in the body, most determinants present in the body can be found in measurements of the blood. The advantages of blood monitoring include the fact that relatively small interindividual variations of those blood components affect blood levels and that a simple sampling technique provides little opportunity for sample contamination. Disadvantages are that the sampling technique is invasive and requires medical personnel, the samples deteriorate when not properly stored, and meticulous cleanup of equipment is required. For biological monitoring blood collected from an arm which has been in direct contact with a chemical may reveal a higher content of the chemical than blood drawn from elsewhere in the body. There is also a difference in concentrations between arterial and venous blood samples due to the pulmonary uptake or washout of volatiles. Circulation of blood fulfills two functions: supply oxygen to the body and remove metabolic end products and excessive heat from every cell. Blood itself is a suspension of cellular elements in an aqueous solution of electrolytes and some nonelectrolytes and macromolecules. Collection of venous blood is most common, but collection of capillary blood from fingers or ear lobes is also feasible in field conditions. Precautions to take when sampling include the washing of the site with isopropyl- alcohol, collection of blood into an acid washed, heavy metal free glass container, mixing of samples before an aliquot is taken for analysis without vigorous shaking, and the careful selection of analytical methods.