Abstract: In the present paper, extractable nuclear antigen （ENA） isolated from fresh calf thymus was reported. RNP and Sm antigens, the components of ENA, were identified by using positive antiRNP and anti-Sm antibodies. With RNP and Sm antigens, the presence of serum anti-RNP and anti-Sm antibodies was detected by counterimmunoelectro phoresis. As the result these antibodies were absent in all 100 normal human sera. The observation of anti-RNP and anti-Sm antibodies in sera of patients with diseases will be present later.