1. Binding of branched-chain 2-oxo acids to defatted bovine serum albumin was shown by gel chromatography and equilibrium dialysis. 2. Equilibrium-dialysis data suggest a two-side model for binding in Krebs-Henseleit saline at 37 degrees C with n1 = 1 and n2 = 5. Site association constants were: 4-methyl-2-oxovalerate, k1 = 8.7 x 10(3) M-1, k2 = 0.09 x 10(3) M-1; 3-methyl-2-oxovalerate, k1 = 9.8 x 10(3) M-1, k2 = 0.08 x 10(3) M-1; 3-methyl-2-oxobutyrate, k1 = 1.27 x 10(3) M-1, k2 = less than 0.05 x 10(3) M-1. 3. Binding of 4-methyl-2-oxovalerate to defatted albumin in a phosphate-buffered saline, pH 7.4, gave the following thermodynamic parameters: primary site delta H0(1) = -28.6kJ . mol-1 and delta S0(1) = -15.2J . mol-1 . K-1 (delta G0(1) = -24.0kJ . mol-1 at 37 degrees C) and secondary sites delta H0(2) = -25.4kJ . mol-1 and delta S0(2) = -46.1J . mol-1 . K-1 (delta G0(1) = -11.2kJ . mol-1 at 37 degrees C). Thus binding at both sites is temperature-dependent and increases with decreasing temperature. 4. Inhibition studies suggest that 4-methyl-2-oxovalerate may associate with defatted albumin at a binding site for medium-chain fatty acids. 5. Binding of the 2-oxo acids in bovine, rat and human plasma follows a similar pattern to binding to defatted albumin. The proportion bound in bovine and human plasma is much higher than in rat plasma. 6. Binding to plasma protein, and not active transport, explains the high concentration of branched-chain 2-oxo acids leaving rat skeletal muscle relative to the concentration within the tissue, but does not explain the 2-oxo acid concentration gradient between plasma and liver.