Recent evidence suggests the existence of several endogenous Na+, K+-ATPaseinhibitors in mammals. Previously, we have shown that the amphibian Na+, K+-ATPaseinhibitor marinobufagenin (3,5-dihydroxy-14,15-epoxy bufodienolide) acts as a vasoconstrictor in isolated rat and human arteries. Mammalian plasma was shown to contain marinobufagenin-like immunoreactive material, which is responsive to saline volume expansion. The present study describes purification of a bufodienolide, which is similar to marinobufagenin, from the urine of patients after acute myocardial infarction with the use of thin-layer chromatography and reverse-phase high-performance liquid chromatography (HPLC). The purified substance cross-reacted with marinobufagenin antibody, demonstrated maximal UV absorbance at 300 nm characteristic of bufodienolides, and eluted from HPLC columns with the same retention time as marinobufagenin. Mass spectrometry of purified material revealed the presence of a substance indistinguishable from amphibian marinobufagenin and having molecular mass of 400 D. The present studies show that one of the human digitalis-like factors may have a bufodienolide structure and is likely to represent marinobufagenin or its isomer, and they suggest a role for this substance in the pathogenesis of myocardial ischemia. (Hypertension. 1998;31:1097-1103.)