The distribution of 75Se among various electrophoretic serum protein fractions was studied at different time intervals after the administration of the isotope to chicks. Within the first 2 h after crop intubation with H275SeO3, 33%–44% and 20%–24% of the serum protein radioactivity was located in the α2 and α3 globulins respectively. The percentage of serum protein radioactivity bound to the γ globulin increased at a rapid rate during the first 24 h. During the subsequent 24- to 173-h interval, 52%–70% of the 75Se was carried by the α2 and γ globulin fractions. The development of pectoral myopathy, as the result of a concomitant dietary deficiency of selenium, vitamin E, and cystine, had no effect on the time distribution of 75Se among the various serum protein fractions. Elevation of the dose carrier level of selenium from 2.1 μg to 300 μg caused a high proportion of the 75Se activity to become bound to the albumin during the first 28 h. When the serum proteins were doubly labeled with 75Se and 14C, the α2 and α3 globulins turned over 75Se more rapidly than the 14C label during the first 12 h after dosage. The data indicate that initially selenium is labile-bound to the α2 and α3 globulins and to albumin if the selenium dose level is high. Later, the 75Se complexed with the globulins is turned over at a slower rate than the 14C label as a preponderance of a more stable complex of protein-bound selenium develops. At this later time most of the 75Se activity is carried by the γ and α2 globulins.