Investigation of factors influencing the δ-aminolevulinic acid dehydratase (ALAD) for monitoring lead-blood concentration was performed with a view towards elimination of procedural artifacts. Using the optimum procedure the in vitro effects of the monovalent cations of Li, Na, K, Cs, NH4, Tl, Hg, and Ag on the pH-activity profile of ALAD from human blood over a seven day period from the date of blood acquisition were investigated and displayed. Several interferences, most notably Hg(II) from HgCl2, were identified and it was found that Cu(I) and Ag(I) deactivated the enzyme while others showed negligible effects on the pH-activity profile and especially on a predefined pH-activity ratio of activities at pH values 6.3 and 7.2 .