In order to simplify FEP (free erythrocyte protoporphyrin) test, we studied on the Piomelli's method, specially on the preparation of standard solution and on the role of celite-saline. N,N-dimethylformamide (DMFA) was used as the solvent of protoporphyrin standard (protoporphyrin IX dimethylester, Sigma Chemical Co.). The stock solution was stable for over 6 months at 4 degrees C. To examine the recovery and extraction efficiency, the stock solution (PP) diluted with DMFA was also added to blood. Ten microliter of DMFA added to 40 microliter blood had no effect on the FEP extraction. In the absence or at low concentrations of protein components, fluorescence intensity of added protoporphyrin extremely decreased in the extract with celite-saline. It seems to be due to the adsorption of protoporphyrin to celite. In the presence of proteins, there was no difference between fluorescence intensity of protoporphyrin with celite-saline dilution extraction and that with distilled water dilution extraction. The FEP levels in 128 lead workers (blood lead levels, 4.9-80.8 micrograms/100 g) with celite-saline extraction were almost the same as with distilled water dilution extraction. Thus celite-saline dilution was able to be displaced by the distilled water dilution, and the extraction procedure was slightly simplified. With our method, the average recovery was 93% and extraction efficiency was 80-90%.