Biochemical biomarkers measure the exposure of organisms to environmental chemicals. They can also measure the toxic effect, e.g. where they are based on molecular mechanisms which underlay toxicity. Ideally, biomarkers should be sensitive, specific, simple to use and suitable for the assay of material obtained by non-destructive sampling procedures (e.g. of blood). There has been commendable progress in the development of several different types of biomarker assays. The measurement of inhibition of serum 'B' esterases to monitor exposure of birds to organophosphorus insecticides.The measurement of DNA damage caused by aromatic hydrocarbons. The measurement of disturbances to the transthyretin-retinol binding protein complex caused by a metabolite of 3,4,3',4',tetrachlorobiphenyl. The measurement of precursors of clotting proteins in blood following the inhibition of the Vitamin K cycle by anticoagulant rodenticides. Out of these, the first is only a biomarker of exposure but the remaining three are, in principle, biomarkers of toxic effect as they all represent measures of molecular mechanisms which underlay toxicity. Biochemical biomarkers have considerable potential for measuring effects of chemicals under field conditions--especially where carefully selected combinations of them are used.