We have produced the human class II histocompatibility protein, HLA-DR1, as a soluble, secreted glycoprotein in insect cells infected with baculoviruses carrying truncated α and β subunit genes. The peptide-binding site is empty, and the empty molecules are fully competent to bind antigenic peptide. We used the empty molecules to measure an intrinsic rate for peptide association, and to investigate the role of peptide in stabilizing the class II structure. Peptide binding kinetics for the empty molecule are only 10-fold faster than for peptide exchange into an occupied site, suggesting that a conformational change may accompany peptide binding. The native αβ heterodimer assembles in the absence of antigenic peptide, but peptide binding stabilizes the empty heterodimer against aggregation and against SDS-induced denaturation.