Peptides immunoreactive with antibodies to native proteins, and which have at least two cysteine residues that contribute to mimicking an epitope of the protein, are prepared with the cysteine thiol groups protected. When deprotected, the peptides have enhanced immunoreactivity. The peptides are particularly useful for detecting antibodies or antigens associated with retroviruses, including the clinically important lymphotropic retroviruses HIV-1, HIV-2, HTLV-I, and HTLV-II.