A systematic survey of blood samples from 174 industrial workers occupationally exposed to lead fumes and lead dust, revealed some poor correlations between the lead (Pb) level and the free erythrocyte protoporphyrin (FEP) level. In addition to those workers who had an expected correlation (normal Pb-normal FEP or elevated Pb-elevated FEP) there were two groups with atypical correlations. In one group a normal Pb level was associated with an elevated FEP level and in the other group an elevated Pb level was associated with a normal FEP level. This shows that neither the Pb nor the FEP test can substitute for the other. Since the individual FEP response to Pb absorption is unpredictable, both tests should be carried out routinely. This would make it possible to determine simultaneously the degree of absorption (Pb level) and the degree of intoxication (FEP level) and would improve significantly the assessment of undue lead absorption of occupationally exposed workers. If, however, only one of these tests is used for screening the other test should always be carried out as a confirmatory test when the screening detects an elevated Pb or FEP level.