We describe an assay for delta-aminolevulinic acid dehydratase (EC 4.2.1.24) activity. Radioactive [14C]porphobilinogen, formed by action of this enzyme on [14C]delta-aminolevulinic acid, is purified by passage through an ion-exchange chromatographic column before measurement with a liquid scintillation counter. The radioactive substance in the final solution was identified as solely [14C]porphobilinogen by paper-chromatographic analysis. The present assay procedure requires only a 0.1-microL sample of blood and is about 100-fold more sensitive than the conventional colorimetric methods involving Ehrlich's reagent. Using this method, we found that activity of this enzyme in the bone marrow of rats decreases abruptly and sharply two weeks after birth.