The purpose of this experiment is to clarify the additional effect of antibiotics for the chicken breeding feed. This experiment was carried out with 6,560 individuals which were collected from four farms, Dugil, Hyubhoi, Sanggog and Kumsan. The duration of this experiment was 6 months from July to December in 1981. 1. Body weight and body gain. (1) The range of the body gain was 1.91-7.60% The most effective sections was T$_3$, and the effect was T$_2$, T$_4$, T$_1$, T$\sub$5/ in order. (2) The environmental sanitation of the Sanggog farm was not so good. In this farm the body gain was severely occurred in T$_3$ and T$_2$ sections. Mere gain obtained from T$\sub$5/ section. (3) The environmental sanitation was good in Dugil, Hyubhoi and Kumsan. A little body gain was occurred in these farms, but differences were shown according to the place or breeds. 2. Amount of feed intake feed conversion ratio. (1) Amount of feed intake was increased 0.13-3.19%, and feed conversion ratio was improved 1.68-4.62% compared with control. T$_3$ section showed high percent and T$_4$ T$_1$, T$\sub$5/, Is, C in order. (2) The keeping method of the chicken breed in Hyubhoi was floor brooding. From Hyubhoi and Sanggog farms, the efficiency was increased 9.4-9.8%. (3) From Dugil and Kumsan, the rate of improvement was low. 3. Mortality : Mortality was merely decreased in Sanggog and Kumsan farms which the sanitary and fowl caring conditions had been had, by additional antibiotics. But there was no worthy effect. 4. The ratio of carcass and the degree of skin coloration. The additional antibiotics in feed had no effect on the ratio of carcass but had the influence on the yellowish skin coloration. Skin coloring was good in T$\sub$7/ section and T$_1$, T$\sub$6/, T$_4$, C, T$_3$, T$\sub$5/, T$_2$ in order. 5. Economical efficiency (1) The addition of the antibiotics in feed increased 3.76-10.95% of gross earning. The highest earning was occurred in T$_4$ section and T$_3$, T$_1$, T$_2$, T$\sub$5/, C sections in order. (2) The earning rate was influenced by the antibiotic products. (3) In Dugil and Kumsan farms, there was no difference in earning, however, in Hyubhoi and Sanggog farms there was wide difference in earning. Conclusionally, the addition of antibiotics in chicken breeding feed is necessary. Especially in bad condition of environmental sanitation and floor brooding, the effect of additional antibiotics is clear. Selection of antibiotics from various kinds, maker's confidence and quality problems are important.