The present invention comprises novel and modified peptides capable of inducing an HIV-1 specific immune response without antagonizing the cytotoxic T-cell activity in order to achieve an effective prophylactic and therapeutic vaccine against HIV. The peptides are based on conserved regions of HIV gag p24 proteins. Antigens in free- or carrier-bound form comprising at least one of the said peptides, vaccine compositions containing at least one of the antigens, immunoassay kits and a method of detecting antibodies induced by HIV or HIV specific peptides using such antigens, are described.