The structure of two T-immunogenic peptides, one from the gag p24 protein of the human immunodeficiency virus, the other from the 11.1 gene product of Plasmodium falciparum, was studied by circular dichroism spectroscopy in various pH and solvent conditions. Although both sequences are predicted to adopt an alpha-helical conformation and one of them is a repeat of a perfect alpha-amphipathic sequence pattern, these two peptides exhibit a strong propensity to adopt an extended, turn or aperiodical conformation in solution.