The effect of steroid hormones on the staining intensity of Es-1 region of the zymogram of chicken plasma alpha-naphtyl butyrate esterases was investigated. The Es-1 region is the most anodally migrating region and it's types are found to be controlled genetically (KIMURA, 1969). The staining intensity of this region of laying hens is usually too weak to determine the Es-1 phenotypes. Intramuscular injections of estradiol benzoate to cocks resulted in a diminished staining intensity of the Es-1

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... region. Progesterone and testosterone enhanced the intensity. After the injection of these two hormones to laying hens with a daily dose of 1mg. per kg. of body weight for five days, the Es-1 region showed a strong staining intensity which is comparable to that obtained in cock's, while egg laying were not affected by these injections. (Japan. Poultry Sci., 7, 83, 1970)