The erythrocyte pyrimidine 5'-nucleotidase activity (P5N) was measured in workers exposed to lead, cadmium or inorganic mercury and the effect of lead on this enzyme activity was compared with that on the classical biological parameters (ALAD, ZPP, ALAU), reflecting the interference of this metal with the heme biosynthesis pathway. P5N is inhibited by lead but not by cadmium and mercury. The correlation coefficient between log P5N and lead in blood (PbB) amount to 0.79 (n = 278; PbB ranging from 4.9 to 73.5 micrograms/100 ml). The response of P5N to lead closely parallels that of ALAD.