The CD38-CD9-MHC class II complex is present in membrane rafts. Purified monocytes were lysed in 0.5% Triton X-100 and subjected to sucrose density gradient ultracentrifugation. Raft (fractions 2-4) and nonraft (fractions 7-9) fractions were collected and lysed in CHAPS before immunoprecipitation. After electrophoresis under nonreducing conditions and transfer to a nitrocellulose membrane, Western blots were performed with biotin-labeled CD38 (Leu 17), CD9 (Syb.1), or anti–HLA-DR (DA6.147) mAbs.