I^(131) administered one time into the thigh in dose of 5.0 mci. per Kg of body weight, and iodocasein of 0.25 percent of the basal ration and tapazol mixed with the basal ration in dose of 3.0 ㎎. Per ㎏ of body weight during all the experimental period, were tested to measure their influence on growth, daily gain, carcass yield, carcass percentage and organ weight of three weeks old 80 male white leghorn cockerels divided into 4 groups containing the control group. Carcass yield, carcass percentage and organ weight were observed 205 days after the beginning of administration. The results obtained are as follows; 1. Body Weight at the end of experimental period in tapazol group was the most 1641.5±41.5g (control: 1567±43.5g) showing the significance with p$lt;0.01, and iodocasein group with 1443.0±41.8g and I^(131) group with 1167.0±39.0g apparently lighter than control groin (p$lt;0.01). While I^(131) group showed the most rapid grown week after administration, and rapid growth in iodocasein group and slow in tapazol with no significance were observed until 4 weeks after the administration. 2. Daily gain and carcass yield showed a tendency to similar changes. Tapazol group was higher than the control, and iodocasein group and I^(131) group were lower than control showing no significance between the control and tapazol group, the control and iodocasein group. 3. Carcass percentage showed the significance with p$lt;0.01 among the groups and were increased at the rank of I^(131), tapazol, control and iodocasein group, showing no significance between neighbouring groups. 4. The viscera fat showed notable difference (p$lt;0.01) among the groups: 30.29±4.27 g in I^(131) group, 10.77±1.34 g, in tapazol group, 4.21±0.24 g in control group and little in iodocasein group. 5. Head weight was the most in tapazol group and the lightest in I^(131) group. There was no significance among the groups except I^(-131) group. 6. Weights of gizzard, heart, and spleen mere the most in the tapazol group and the lightest in I^(131) group, generally being in proportion to body weight. 7. Liver weight showed significance with p$lt;0.01 among the groups, and were heavy as the arnk of tapazol group, I^(131) group, iodocasein group and control group, showing no significance between neighbouring groups. 8. Comb and wattle showed a similar tendency and were decreased obviously in I^(131) group (p$lt;0.01). There was no significance among other groups except I^(131) group. 9. Cloacal thymus was atrophied and degenerated in I^(131) group, and its weight was in the rank of iodocasein group, tapazol group and control group, showing the significance with p$lt;0.01.