Molecular mimicries exist between EBV encoded proteins & tissue proteins, investigated in the etiopathogenesis of rheumatoid arthritis: glycine/alanine pereat in EBNA-1, sequence homologies between gp110 and HLA-DR4 molecules, & between EBNA-5 and type II collagen. T-cell proliferation is used in tests of the significance of these mimicries. Cytokine modulation of the T cell responses is studied. IL-6 is a co-stimulant for T cell proliferation CD8+>CD4+.