A recombinant antigen produced in CV-I cells infected with vaccinia virus vC5 carrying the HIV-I gag gene was used to test sera. This antigen (rp50) reacted with 95 serum specimens shown to have anticore antibodies by immunoblot based on natural HIV antigens. Six sera from blood donors positive in ELISA contained antibodies to p17, p24, or p55 by natural antigen-based blot. All these sera did not react with rp50. These patients did not belong to any known risk groups and showed no dynamics in the immunoblot pattern. We consider the reaction of their serum samples as false positive. We believe that the recombinant antigen rp50 may be used for verification of positive ELISA results.