The antigenic specificities of RNA reactive IgG autoantibodies from three individual human SLE sera and pooled NZB/NZW mouse serum were studied by competition reactions with synthetic co-polymers and heteropolymers of varying nucleotide compositions. Although minor differences in the individual binding patterns were noted, the RNA reactive immunoglobulins showed a clear preference for G, C and G,C,U containing RNA sequences. All three SLE sera had Sm or RNP activities with rabbit thymus extractable nuclear antigen (ENA). Conversely, all SLE sera that were selected on the basis of their positive ENA reaction could also bind free, single stranded RNA. These results suggest that the autoimmune response to RNA in human and murine SLE is restricted to one or a very few antigenic determinants. They also raise the possibility that autoantibodies to native RNA in SLE sera are partly responsible for ENA reactivity.