Bovine sera from venous blood were maintained at about 40[degree]F for 1 to 4 days before the detn. of serum precipitable and butanol extractable iodine. The serum precipitable iodine (SPI) and butanol extractable iodine (BEI) values are the avg. of duplicate analyses; the standard deviation of the duplicates from the averages of 30 BEI detns. was 0.28 [mu]g.%. Four comparisons (10 sera in each comparison) were made of the SPI content of Jersey and Aberdeen Angus sera. The avg. SPI of the Jersey sera was 5 [mu]g.%, whereas that of the Aberdeen Angus sera was 4.2 [mu]g.%. In a comparison [mu]of Jersey and Brown Swiss sera the former was found to be higher in PBI than the latter, 4.6 and 3.7 ug%, respectively; the avg. BEI of the sera, however, was identical (2.5 [mu]g.%). The range of SPI for Jersey and Brown Swiss heifers was so great that no comparison was justified; the avg. BEI was 2.5 [mu]g.% and 3.9 [mu]g.%, respectively.