We developed a simple and reliable method for determining delta-aminolevulinic acid (ALA) in plasma (ALA-P) by fluorometric high-performance liquid chromatography. The method was applicable to 100 microliters of plasma and the mean +/- standard deviation for analytical recovery was 100.4 +/- 2.6% when 50 micrograms/l of ALA was added to 14 plasma samples. The detection limit for ALA-P was 2.0 micrograms/l (signal-to-noise ratio was 5) and the standard curve was linear in a wide range up to 400 micrograms/l. The ALA-P level for 26 unexposed male adults was 8.6 +/- 1.3 micrograms/l. In 72 male lead workers, logarithm of ALA-P concentration was significantly correlated with blood lead concentrations (Pb-B) of 2.5-115.4 micrograms/dl (r = 0.924, p < 0.001). Even at the Pb-B level less than 15 micrograms/dl, significant linear relationships were found between Pb-B and log ALA-P (r = 0.632, p < 0.01), between Pb-B and delta-aminolevulinic acid dehydratase activity (ALA-D) (r = -0.573, p < 0.05), and between ALA-D and ALA-P (r = -0.765, p < 0.001). It is suggested that ALA-P determined by the present method is a useful indicator of biological effects of lead, especially in the wide range of Pb-B level.