To provide more sensitive and convenient methods for the detection of equine infectious anemia virus (EIAV), we developed an enzyme-linked immunosorbent assay (ELISA) employing the EIAV gag precursor (Pr55(gag)) produced by using recombinant DNA techniques. The antigenic reactivity of the recombinant EIAV Pr55(gag) was found to be equivalent to that of the virion p24(gag) and elicited high-titered antiserum in rabbits. When a large number of horse sera were analyzed for the presence of antibodies to EIAV by this ELISA, a radioimmunoassay for EIAV p15(gag), or the standard agar gel immunodiffusion test, there was 98.7% concordance among the assays. By using the ELISA it was possible to specifically detect antibodies earlier after experimental infection of horses with EIAV than with the other two tests. A competition ELISA developed in order to detect EIAV gag antigens was found to be approximately 15 times more sensitive than the radioimmunoassay for EIAV p15(gag). Antigens of other animal lentiviruses as well as those of the prototype oncovirus failed to compete in this assay.