A fluorometric method for the determination of naproxen in serum, albumin solutions and protein free buffer solutions is described. The detection limit is about 10 ng/ml. Furosemide, thiopental and salicylic acid did not show any disturbing fluorescence while phenprocoumon did. The in vitro binding of naproxen in albumin solutions and serum was studied by equilibrium dialysis. A small but significant increase was found in the percentual binding in albumin solutions as compared to serum. The percentual binding was not affected by changes in the pH from 5--8. By fitting the binding data to a model assuming two classes of binding sites, association constants and binding capacities were determined. A very high affinity and a high capacity were found. The association constant for the first class of binding sites was higher for human serum albumin than for serum (8.5 versus 5.3 . 10(6) M-1). The difference in the protein binding to the first class of binding sites in human serum albumin and serum can be explained by the existence of a competitive inhibitor in serum.