In lead-exposed workers many factors affect the relationship between the levels of lead and of zinc protoporphyrin (ZPP) in blood. When the zinc protoporphyrin level is used to assess the lead in the blood level, the analytical accuracy of the ZPP and the PbB determinations should be known. Also the variability of lead exposure over time is an important parameter of this assessment. The absolute and relative precision of ZPP measurements with two brands of ZPP meters (AVIV and ESA) were compared. The absolute precision of the ZPP measurements is not constant for the AVIV meter, but is constant for the ESA meter. The relative precision for both brands of meters decreases with increasing ZPP levels. Between the AVIV and the ESA meters, a significant difference in response, irrespective of the kind of anticoagulant used, was observed. The regression equations between PbB and ZPP levels were studied. Different factors can affect the relationship between PbB and ZPP, e.g. variability in lead exposure, the time-lag between the increase of PbB and ZPP, and the precision and accuracy of PbB and ZPP measurements.