AIDS vaccines now use a truncated version of 86 residues of the Tat protein related to the HIV-1 HXB2 strain predominant in Europe and North America. We compared antibodies raised in rabbits using a B subtype short Tat HXB2(86) and a full-length Tat HXB2(100). Serum against HXB2(86) recognizes only B and D subtypes while serum against HXB2(100) recognizes B, D, and C subtype variants. Conformational epitopes appear to be involved in the capacity of anti-Tat HXB2 sera to recognized non-homologous Tat variants. A linear B-epitope identified in sequence 71-81 in HXB2(86) disappears in HXB2(100), which has a new linear B-epitope identified at the C-terminus. Anti-HXB2(100) serum has a higher titer in neutralizing antibody against homologous and non-homologous variants compared to anti-HXB2(86) serum. We suggest that a Tat vaccine should contain a Tat variant with regular size, up to 99-101 residues now found in the field.