The decrease of pyrimidine 5'-nucleotidase (P5N) in human erythrocytes, detected in persons occupationally exposed to lead (Pb), is not an in vitro artefact resulting from the release of lead during erythrocyte hemolysis but is a true reflection of the enzyme activity in vivo. Like Pb, mercury (Hg) and cadmium (Cd) ions exhibit an important in vitro inhibitory action on the P5N enzymic activity. However, when the metals are preincubated as metallothionein complexes with P5N preparations, their inhibitory power disappears.