Anti-double-stranded (ds) DNA autoantibodies are considered as pathogenic in systemic lupus erythematosus (SLE). Anti-DNA antibodies have been shown to be released by B cells from lupus mice or patients in vitro. A monoclonal anti-DNA antibody (PME 77) specific for dsDNA has previously been shown to react with a cell-surface protein called LAMP (lupus-associated membrane protein), which is present on the cell surface of various cell types involved in SLE pathogenesis. Using an immunoreplica analysis technique, we show here that spleen cells from MRL/Mp-lpr/lpr lupus mice, cultured in vitro, spontaneously produce anti-LAMP IgG antibodies. Conversely, anti-LAMP antibodies were not detected in spleen-cell culture supernatants from nonautoimmune CBA/Ca mice. Taken together with our previous reports, this result adds a new argument for a pathogenic role of anti-LAMP autoantibodies in SLE.