We developed an automated HPLC method for determining erythrocyte protoporphyrin. After a single extraction with N,N-dimethylformamide (DMF), zinc protoporphyrin IX (ZP) and protoporphyrin IX (PP) were separated by the column packed with a new type of reversed phase silica (capcell type) within 5 min. The column life was markedly extended to facilitate sample injection for about 1,000 times. The concentrations of ZP or TP (total protoporphyrin = 0.9 ZP + PP) determined by the present method correlated well with those determined by the conventional methods, i.e. by hematofluorometer (HF) or acid extraction method (FEP). In 150 workers exposed to lead, the ratio of 0.9 ZP/TP varied between 0.43 and 0.99 with the ratio being less than 0.6 in only 5 workers. The correlation coefficient between Pb-B and ZP was significantly higher than that between Pb-B and PP. Both anticoagulants, heparin and EDTA, could be used in the present method as well as in HF and FEP methods. Reference values for TP by HPLC were between 31.2 and 120.9 micrograms/dl RBC. Samples stored at 37 degrees C for 3 days were also used in the present method and the same results were obtained as in the samples stored at 4 degrees C.