The high-affinity binding of perazine to human serum-protein (non-albumin binding) was previously investigated by gel-chromatography. The immunoelectrophoretic identification of the binding agent as alpha 1-acid glycoprotein is described here. It was demonstrated by equilibrium dialysis that the average free fraction of 3H-perazine added to 22 sera from patients before neuroleptic treatment was 3.67 +/- 0.42%, and that there was a significant correlation between the alpha 1-acid glycoprotein content and the free fraction in these serum samples. This result is in accordance with what others have found for imipramine. It is suggested that the nature of this binding should be studied in more detail, since specific binding to alpha 1-acid glycoprotein may be related to the receptor binding of perazine and possibly other drugs.