Antisera to bovine serum albumin were raised in rabbits. Studies of immunological activities of some of the fragments from bovine serum albumin and other species of serum albumins showed that immune response is dynamic rather than static. The results of immunological activities of fragments of bovine serum albumin were analyzed carefully and 5 regions of the molecule were predicted to be antigenic determinants. Synthesis of these regions confirmed this prediction. These results suggest that the serum albumin molecule has at least a partially repeated antigenic structure.