Sakai's method has been known as the simplest one for determination of erythrocyte pyrimidine 5'-nucleotidase (P5N) activity using high performance liquid chromatography(HPLC). However the drawback of the method is that it is difficult to wash the erythrocyte for isolation. To search for the simpler method, we compared Sakai's method with other methods using whole blood treated with heparin and concanavalin A or whole blood treated with EDTA-2K instead of washing the erythrocyte. The mean concentrations of lead in blood samples collected from 44 male and 16 female workers who are healthy without any exposure to lead in their workplace were 4.30 +/- 1.31 microgram /dl (mean +/-standard deviation), which were measured by frameless atomic absorption spectrophotometer. Erythrocyte P5N activities were measured by 3 methods; Sakai's method(Method I), using whole blood treated with heparin and concanavalin A (Method II), and using whole blood treated with EDTA-2K (Method III). The results were obtained as follows ; 1. The mean of erythrocyte P5N activity by Sakai's method(Method I) were 12.7 +/-2.47 amole uridine/hr/gm of Hb. 2. The mean of erythrocyte P5N activity by the method using heparinized whole blood treated with concanavalin A(Method II) were 13.1 +/-2.41 micromole uridine/hr/gm of Hb. 3. The difference of mean erythrocyte P5N activity between Method I and Method was not significant. 4. The erythrocyte P5N activity by the method using whole blood treated with EDTA-2K (Method III) was significantly different from Method I. We thought that omission of incubation period which was required on Method III using EDTA-2K caused the difference between Method I and Method III. 5. Simple linear regression equation for erythrocyte P5N activity between Method I (Y) and Method II(X) was significant: Y = -0.012 + 0.9724 X. These results suggest that the method using whole blood treated with heparin and concanavalin A is simpler to examine the erythrocyte P5N activity as a biological indicator of lead intoxication than Sakai's method.