In laying hens, immunoglobulin G (IgG) is transferred from maternal body to the egg yolk and termed as immunoglobulin Y (IgY). The goal of this study was to determine the effects of aging and estrogen treatment on the concentration of IgY in egg yolk. The concentration of yolk IgY and plasma IgG of young laying (approximately 170-day-old), middle-aged laying (approximately 250-day-old) and old laying (approximately 575-day-old) hens as well as old laying hens (approximately 575-day-old) treated with diethylstilbestrol (DES, an estrogenic compound) for 7 days were determined by ELISA. Immunoglobulin Y concentration was significantly greater in young laying hens than in middle-aged and old laying hens, in contrast, plasma IgG concentration was greater in old laying hens than in middle-aged and young laying hens (P<0.01). The concentration of yolk IgY and plasma IgG was significantly increased after DES treatment as compared with pretreatment (P<0.01). Significant differences were not observed in the concentration of yolk IgY and plasma IgG between pre- and post-oil treatment of hens (control birds). These results indicate that the concentration of IgY decreases with aging of hens unlike plasma IgG concentration, and the concentration of IgY is significantly affected by estrogen treatment.