We developed an antibody detection enzyme-linked immunosorbent assay (ELISA) using recombinant surface (SU), transmembrane (TM) and capsid (CA) antigens of feline immunodeficiency virus (FIV) expressed in Escherichia coli. The three antigens were tested with sera collected from experimentally infected cats in order to follow the course of seroconversion and of the antibody levels throughout the infection. An early and marked increase of TM antibodies was observed. Antibodies to TM were demonstrated at high levels throughout the observation period. The immune response to SU and to CA was less pronounced and in some cats the level of antibodies to SU and CA tended to decline 6 months after infection. In addition, 413 FIV negative and positive cat sera were tested in order to define for each antigen the diagnostic sensitivity, specificity and efficiency. TM showed the highest diagnostic sensitivity (98%) while its specificity was 97%. Its diagnostic efficiency of 97% was better than that of SU and CA and exceeded that of tests utilizing conventionally grown and gradient purified FIV. Therefore, recombinant TM can be considered a very important antigen for FIV ELISA testing. An interesting perspective is offered in the combination of TM with other recombinant antigens in a dot assay form.