HLA-eluted platelet transfusions may be feasible for transfusion in HLA-alloimmunized refractory patients, but the reduction in HLA expression after elution is variable. Using different anti-HLA antibodies directed against the HLA class I complex, the heavy chain, beta 2-microglobulin (beta 2M) and the intracytoplasmatic part of the heavy chain, we investigated factors which influenced the HLA reduction by flow cytometry. Reduction of HLA expression by acid treatment resulted in an almost complete removal of HLA class I molecules from the site of the intracytoplasmatic part of the heavy chain. Re-expression of HLA heavy chain, beta 2M and some of the trimolecular complex (comprised by beta 2M, heavy chain and peptide) occurred within 4 h. This was presumably the result of adsorption from the supernatant (eluate or plasma). After immediate removal of the eluate and incubation in plasma-free supernatant no re-expression of HLA occurred after a 4 h interval.