Ithas been known for years that free erythrocyte protoporphyrin(FEP) 1 concentrations increase in lead poisoning, iron-deficiency states, and protoporphyria, an uncommon inborn error of porphyrin metabolism. The complex nature of the older procedures for extracting protoporphyrin IX (PROTO) in multiple steps from blood (1, 2), the lack of readily available standards from commercial sources, and other factors have largely limited PEP measurements in the past to experimental work and clinical investigation. The purpose of this report is to describe a simple and reproducible micro-scale fluorometric procedure, which can be performed on 20 tl of blood with use of either a filter fluorometer or a spectrophotofluorometer and with standard disposable glassware and commercially available high-purity calibrationmaterials and reagents. It is based on the classicalethyl acetate/aceticacid extraction technique of Schwartz et al.(1), as are other recently published micromethods(3-5).