The level of free tryptophan (Trp) and its metabolites in serum appears to be related to some pathologic states, such as chronic renal failure and neuropsychiatric disorders, so that a precise characterization of tryptophan binding to serum albumin is of interest. In the present paper, the binding of L-tryptophan to defatted human serum albumin at 37 degrees C and at pH 7.4 was studied by means of equilibrium dialysis. The competition between L-tryptophan and serum solutes extracted from uremic patients undergoing hemodialysis, before dialysis treatment, was also investigated. Solutes were extracted from uremic pools of sera using two different deproteinization methods: serum ultrafiltration and heat denaturation of serum proteins followed by ultrafiltration. We found 1.10 +/- 0.03 binding sites for Trp to defatted albumin with an association constant 11.37 +/- 1.03 x 10(3) M-1. The competition experiments suggested that the number of Trp binding sites were not significantly modified by the addition of solutes obtained with the method of ultrafiltration with respect to the binding of L-tryptophan to albumin in the absence of competitors, while their affinity constant was markedly reduced (2.66 +/- 0.18 x 10(3) M-1). Moreover, a significant reduction of the affinity constant was observed when competitors for Trp were obtained using heat deproteinization associated with ultrafiltration (1.91 +/- 0.15 x 10(3) M-1 vs. 2.66 +/- 0.18 x 10(3) M-1; P < 0.005). These results might be ascribed to the fact that the last procedure has a higher yield with a more complete liberation of uremic toxins from serum proteins, so that they became probably totally free thus competing at higher extent with L-tryptophan for albumin binding sites.