The boundary patterns of bovine serum obtained at pH 6.0 or 4.0 have been found to vary considerably with the transport media used. In contrast, bimodal patterns have been obtained exclusively at pH 2.5. Bimodality is ascribed to one of the following: (a) interaction between protein and buffer, (b) reversible molecular association reactions between albumins and globulins, or (c) heterogeneity with a spectrum of mobilities. Since, under the experimental conditions, no bimodal patterns were obtained with either purified bovine serum albumin or γ1-globulin, the first possibility can clearly be ruled out. The following facts, in addition to other pieces of supporting evidence, are in favor of the possibility of reequilibration and are inexplicable in terms of merging mobilities: (a) the fluctuation of mobility data and the percentage area fraction data, (b) the bimodal patterns obtained from both the isolated fractions in the rerun experiment, (c) the presence of albumin in both the isolated fractions, (d) the discrepancy between the calculated percentage area of fast peak and the actual measurement when bovine serum albumin was added to the serum, and (e) the presence of a complex boundary in sedimentation diagrams of bovine serum and its fractions obtained in acid buffer, but not in barbiturate buffer. The albumin component has been found to be essential for bimodal behavior from experiments both with serum and with synthetic mixtures of plasma proteins. An interpretation from the viewpoint of the configurational changes of bovine serum albumin at low acid pH is discussed.