Induction of low neutralizing antibody titers in rabbit sera after immunization with gp41 derivatives. After removal of anti-cell antibodies, sera were tested in neutralization assays with two viral strains. Dilutions mediating inhibition of 50% infection (ID50) subtracted by ID50 of pre-bleed sera are shown. Sera of all groups were tested against tier-1 isolates (A) MW965.26 (clade C) and (B) SF162 (clade B). Asterisks mark significant difference between groups (p < 0.05).