The protein binding of propafenone, a Class I antiarrhythmic agent, was studied in vitro using a selective and sensitive electron-capture detection gas-liquid capillary chromatographic assay method developed in our laboratory. The concentration-dependency of the serum protein binding of propafenone was confirmed in vitro by equilibrium dialysis, using serum obtained from healthy human subjects and patients with chronic renal failure. In normal serum the unbound fraction of propafenone was 0.027 at a propafenone concentration of 0.25 microgram.ml-1, 0.041 within the therapeutic concentration range (0.5-2 micrograms.ml-1), 0.138 at a propafenone concentration of 25 micrograms.ml-1, and 0.187 when the propafenone concentration was increased to 100 micrograms.ml-1. There was no evidence of significant concentration-dependent changes in unbound fraction within the propafenone concentration range of 0.5-1.5 micrograms.ml-1. However, concentration-dependent binding was demonstrated at concentrations greater than 1.5 micrograms.ml-1. A high-affinity, low-capacity binding site (K1 = 6.53 x 10(5) l.mol-1; n1P1 = 1.73 x 10(-4) mol.l-1) and a low-affinity, high-capacity binding site (K2 = 8.77 x 10(3) l.mol-1; n2P2 = 8.57 x 10(-3) mol. x l-1) were identified. In pooled uraemic serum the unbound fraction of propafenone was approximately 50% of that of normal serum throughout the concentration range studied (1-5 micrograms.ml-1). In sera from patients with chronic renal failure the increase in propafenone binding ratio or the decrease in unbound fraction was associated with the increase in alpha 1-acid glycoprotein concentrations, and there was a correlation (r = 0.8302) between alpha 1-acid glycoprotein concentration and the propafenone binding ratio.