Under certain pathological conditions the binding of various substances by serum proteins is altered. The plasma concentrations of alpha 1-acid glycoprotein, also known as orosomucoid are reported to be elevated in stressful situation and in certain disease states like acute myocardial infarction. The binding of aminopyrine to serum proteins and alpha 1-acid glycoprotein was determined using equilibrium dialysis. It appears that alpha 1-acid glycoprotein has specific binding sites for aminopyrine. Aminopyrine was also bound to other serum proteins. On addition the results indicate that an increase of alpha 1-acid glycoprotein to concentrations such as those seen in some pathological states does not really alter the percent of aminopyrine bind to serum proteins.