AbstractHere we show that treatment of K562 cells with the phorbol ester TPA induces the down‐modulation of various surface antigens. Among them, the transferrin receptor (TfR), the tetraspanin CD81, and a CD81‐associated protein, CD9P‐1, were unique in that their expression levels were lower after 24 h incubation than after 3 h. We demonstrated that like the TfR, CD81 was internalized at early times, and was less synthesized at latter times. Despite the association of a fraction of the TfR with CD81, these two molecules were subjected to different fates. TPA increased targeting of CD81 and CD9P‐1 into exosomes but strongly reduced the localization of the TfR in these vesicles. Using this model we have shown that a fraction of CD81 and CD9P‐1 in exosomes comes from a surface pool and that these molecules remain associated in exosomes. However, CD9P‐1 could be targeted to exosomes in the absence of CD81 and of another tetraspanin, CD9. The targeting of CD9 into exosomes did not require palmitoylation of the protein. J. Cell. Biochem. 102: 650–664, 2007. © 2007 Wiley‐Liss, Inc.