P15E is a specific sequence among the envelope gene (env)-encoded transmembrane proteins of exogenous and endogenous retroviruses. A synthetic peptide (CKS- 17) that shows homology to this p15E region in several species of retrovirus is known to induce immune abnormalities. In this study, we examined the effect of a synthetic peptide derived from a region of human endogenous retrovirus (HERV) clone 4-1 (lambda4 - 1) similar to sequences of CKS-17 on the induction of systemic lupus erythematosus (SLE)-related immune abnormalities. Our results indicated that this peptide could induce T-cell activation and anergy in normal peripheral blood mononuclear cells, and the peptide could also promote the production of interleukins IL-6 and IL-16. These phenomena are representative immune abnormalities observed in SLE patients. Thus, our findings support the possibility that HERV acts as a pathogen in human SLE.