Abstract: A highly purified, Na.sup.+,K.sup.+ -ATPase inhibitory factor is disclosed herein. The factor does not cross-react with anti-digoxin antibody and exhibits uniquely characteristic UV absorbance maxima at approximately 202-210 nm and at 274-280 nm. The inhibitory factor has a molecular weight of less than 1000 Daltons, is non-peptidic, non-lipidic, and loses activity following charring or alkaline hydrolysis. The factor is useful as a pharmaceutical composition in methods for treating essential hypertension, cardiac malfunction and in regulating angiogenesis and active sodium transport and other conditions. Methods for purifying the factor from tissue and fluid extracts are also disclosed.