A fraction having an antihypertensive effect when administered orally to spontaneously hypertensive rats was partially purified from the heat extract of Enterococcus faecalis FK-23 cells by ultrafiltration and ion exchange chromatography. The antihypertensive effect of the compound from the extract was evaluated in term of its capability of reducing systolic blood pressure of spontaneously hypertensive rats. The absorption maximum at 260 nm, the presence of ribose as the constitutive sugar and the RNase-degradability of the isolated fraction led to the conclusion that the compound contained in the fraction was a ribonucleic acid. The high molecular weight RNA reduced the systolic blood pressure by about 30 mmHg at 3 h after intravenous administration to spontaneously hypertensive rats at a dose of 20 mg/kg. In the case of its oral administration at a dose of 65 mg/kg, the reduction was 15-20 mmHg. However the antihypertensive effect of this fraction was negligible in normotensive rats. These results suggested that the RNA fraction in the soluble heat extract of E. faecalis FK-23 has a selective antihypertensive effect on spontaneously hypertensive rats (SHR).