Abstract: Complexes of HIV Env and Tat proteins are advantageous as immunogens compared to Tat or Env alone, but they may dissociate when combined with a vaccine adjuvant. To avoid dissociation, complexes of Env and Tat are stabilized by the use of covalent cross linking. The extent of cross linking is important to the binding properties of the complexes, and so is controlled to avoid the loss of Env's ability to bind specifically to CD4 and Tat's ability to bind specifically to anti-Tat monoclonal antibodies.