Within the gp41 glycoprotein of the human immunodeficiency virus type 1 (HIV-1) there is a relatively conserved region which appears accessible to the immune system during the course of HIV infection and is recognised by antibody from virtually all patients with AIDS. This region has also been shown to function as a target for human T cells. We have examined synthetic peptides spanning this sequence, between residues 572 and 604, with a view to evaluating their potential as immunogens. Peptides 572GIKQLQARILAVERYLKDQQ591 and 579RILAVERYLKDQQLLGGIWGCSGK601 were good immunogens in two different strains of mice while peptide 576LQARILAVERYLKDQQ591 was an inferior immunogen, and peptide 593LGIWGCSGKLIC604 was non-immunogenic unless coupled to a carrier protein. For both antibody and T cell responses it was apparent that sequences that could function as determinants within one peptide could not do so in the context of a different peptide immunogen. It follows that by judicious choice of immunogen sequence it may be possible to direct the immune response towards a desired fine specificity. Unwanted responses by CD4+ T cells isolated from certain peptide-primed animals were also observed. These T cells showed an unusual reactivity in that they were incapable of recognising their determinant AVERYLKDQQ if it was extended at the C-terminal end with the native sequence and as such would not be expected to recognise the native molecule unless processing created the identical C-terminus.