We have studied the diagnostic efficiency of some biological tests in 59 consecutive patients whom we suspected to be poisoned by lead from drinking water. This diagnosis was confirmed in 30 patients in whom EDTA-promoted urinary excretion of lead was greater than 800 micrograms/24 hr. The measurement of delta-ALA (delta-aminolevulinic acid) dehydrase was the most sensitive test (89.7 p. 100). Sensitivities of protoporphyrin blood level (64.7 p. 100), of serum lead level (46.2 p. 100), of the presence of erythrocytes with basophilic stippling in blood (44.4 p. 100) were lower. The determinations of urinary coproporphyrin and of delta-ALA levels in blood and urine had no practical diagnostic value. We conclude that the best screening test for the diagnosis of lead poisoning by drinking water is the measurement of delta-ALA dehydrase activity. An increase of the EDTA-promoted urinary excretion of lead is required for a final confirmation of the diagnosis.