T cells from rheumatoid synovium have been expandedin vitroas lines and clones using autologous Epstein-Barr virus-transformed stimulator cells. Both lines and clones recognized autologous class II MHC antigens in the absence of defined exogenous antigens i.e. the equivalent of the autologous mixed lymphocyte response. Surprisingly, despite their MHC specificity, several clones expressed CDS rather than CD4, but were not cytotoxic. The function of CD8 + T cells within synovium has not previously been defined; in view of their unusual phenotype, they may exert an immuno-modulating role upon the inflammatory response within the joint, by responding to the high density of class II MHC antigens expressed in the rheumatoid synovium.