It is an expectation on radiocolloid therapy from the standpoint of prevention of side efects, that the administered radiocolloid is not removing in blood stream or in some organs. Old (90)YCl(3) colloid at pH 7.0 (20mμ diameter) and new type of a mixture of YCl(3) and (90)YCl(3) colloid at pH 7.0 (3-15μ diameter) were administered into peritoneal cavity of mice and rabbits. The distributions of administered (90)Y in serum and organs were compared. Also, these two colloids were examined by intralymphatic administration for clinical use, and the distributions were observed in serum and the excretion of urine. 1) (90)Y in peripheral blood by peritoneal administration of mice were higher in old(90)YCl(3) colloid than that of new type of YCl(3) and (90)YCl(3) mixture. The c. p. m. in peripheral blood were generally decreased by addition of YCl(3). Minimum distribution in peripheral blood was obtained by addition of 1mg/ml of YCl(3). 2) The same results were obtained from rabbit experiment. Old (90)YCl(3) colloid was 2 times of c. p. m. than that of new type mixture colloid at 7th day. (90)Y excretion in urine of old (90)Y colloid was 51 times than that of new type colloid after 3 hrs colloid administration. The distributions of old (90)YCl(3) colloid were spleen, kidney, liver, marrow, lung, heart muscle, and thigh, in the orders, 7 days after administration, and that of new type colloid were spleen, liver, marrow, lung, kidney, heart muscle, and thigh. (90)Yc. p. m. of old colloid was 4 times than that of new colloid in kidney, heart muscle and thigh. New colloid was 5 times than old colloid in spleen, liver, marrow and lung. 3) (90)Yc. p. m. in peripheral blood of intralymphatic administration was observed in both colloid, below 1% excretion in urine of administered dose after 3 days. In clinical use, one case was visible reduction of metastatic lymphnode, but clinical effects were not clear, for control patients were end stadium of malignant tumor.