The present invention relates to peptides derived from the superantigen (SAg) ENV protein of the human endogenous retrovirus HERV-K18, and to the use of the peptides in obtaining antibodies which inhibit the superantigen activity of HERV-K18 ENV. The invention also relates to vaccine compositions for treating and preventing disorders associated with the ENV gene product of HERV-K18, for example autoimmune diseases such as insulin-dependent diabetes mellitus (IDDM). A preferred peptide consists of a portion of an N- or C-terminal segment of the HERV-K18.1 ENV protein, as illustrated in FIG. 1A, said N-terminal segment extending from amino acids 22 to 62 of HERV-K18.1 ENV, and said C-terminal segment extending from amino acids 110 to 153 of HERV-K18.1 ENV, wherein the peptide has a length of 6 to 40 amino acids and is capable of giving rise to antibodies which inhibit superantigen activity associated with HERV-K18 envelope proteins.