Antibodies present in sera of patients with autoimmune diseases (systemic lupus erythematosus, mixed connective tissue disease) were reacted with nuclear ribonucleoproteins from normal cells and cells infected with retroviruses. Only antibodies directed against Sm and RNP [ribonucleoprotein] antigens precipitated particles with the definite spectra of small nuclear RNA and proteins. No difference was found between infected and uninfected cells, suggesting that virus replication is dependent on normal cellular functions.