(I) The experiment was designed to study the effects of antibiotic Vitamin B_(12) supplements to chick diet. Many previous experiments show that feeding antibiotics Vitamin B_(12) could increase the body gain, improved feed efficiency and decreased mortality. (2) At the Anyang livestock breeding station, from September, 2. 1959 to November 16, 1959. (8 weeks), the completely randomyzed designed experiment was conducted to study the relationships of the growth rate, efficiency of feed, utilization and mortality between with and without antibiotic Vitamin B_(12) supplements to practical ration. (3) Both Sexes of white Leghorn, day-old-chicks wens reared in bamboo floored electric bulb heated buttery. Feed and water were supplied by self-feeders and wateres regularly. The chicks were leg-banded and devided into 4 lots according to weight and sex. The following antibiotic and Vitamin Bas feeding supplements were used: Aurofac-2A containing 3.6 grams of aureomycin and 3.6 milligrams of Vitamin B_(12) pre pound of supplement. The basal ration was composed with 5% broken rice, 15% wheat, 30% yellow corn, 5% whet bran, 5% wheat middlings, 5% rice bran 20% soyean oil meal, 1.5% clover, 10% silkwarm-pupa oil meal, 1.1% oyster-shell, 0.4% salts and 0.5% vitaton. The Vitamin, mineral and amino acid content of basal diet were calculated and compared with the national Research concils Recommended Nutrient alliwance for Chick. One hundred chicks were selected and divided into 4 lots as shown in the following table lot Ⅰ (C₁) : Basal Diet only lot Ⅱ (T₁) : Basal Diet+aureomycin 10㎎ Vitamin B_(12) 0.01 ㎎/㎏ (Supplied as Aurofac 2A) lot Ⅲ (T₂) : Basal Diet+aureomycin 20㎎+Vit. B_(12) 0.02 ㎎/㎏ (Supplied as Aurefac 2A) lot Ⅳ (T₃): Basal Diet+aureomycin 30㎎+Vit. B_(12) 0.03 ㎎/㎏ (Supplied as Aurefac 2A) Individual chick weight and feed consumption were recorded at AM. 2 on day each for 8 weeks. Mortality was also recorded (4) The growth rate were treated statistically with the completely randomyzed blocks design in the analysis of variance. Efficiency of feed utilization was expressed in units of feed consumed per unit of gain in body weight. (5) Discussion of experimental Results (1) Results that teated by analysis of variance for gain in body weight for 4 weeks and 8 weeks of age, were highly significant (F=0.01) in difference between with and without anticiotic-Vitamin B_(12) feeding supplement. However, when it was compared with by Least significant difference between individual means they were not significant for L.S.D.(t=0.05) with lot Ⅰ and lot Ⅲ at 4 weeks of age, but all other lots were significant for L.S.D.(t=0.05). (2) Efficiency of feed utilization of experimental lot were mere superier than that of control Lot. In number of grams feed consumed per pound gain of experimental lot Ⅰ, Ⅱ and Ⅲ, the feed was saved 126.8, 181.6. and 99.4 grams of feed. in comparing of the control lot respectively. (3) Mortality of control lot was the, highest showing 16 per cent but the experimental lots were about 4 percent in, average.