This standard specifies the procedure to be followed and the equipment necessary for the determination of the activity of delta-aminolevulinic acid dehydrase (ALA-D) in blood, both at physiological and pathological levels, using a spectrophotometric method. The determination of ALA-D in blood is applicable, as a complementary parameter to the level of lead in blood, to the study of populations exposed to metallic lead or its ionic compounds, especially in the case of low levels of exposure, plumbemia between 15 and 35 pg/100 mI (0.72 and 1.69 umol/l) (see section 9.1). As factors that can influence the activity of ALA-D, apart from exposure to lead, the following must be taken into account: zinc counteracts the effect of lead on ALA-D and blood alcohol level and methyl mercury inhibit their activity (see 9.5).