We isolated two candidates for endogenous digitalis-like factors from human urine based on the inhibition of [3H]ouabain binding to intact human erythrocytes. The more-polar ouabain-displacing compound-1 (ODC-1) closely resembled ouabain in biological, physicochemical, and chromatographic properties. Moreover, anti-ouabain IgG dose-dependently neutralized the action of ODC-1. The less-polar ODC-2 behaved identically to digoxin in three analytical high-performance liquid chromatography and thin layer chromatography systems. Fast atom bombardment mass spectrum and proton nuclear magnetic resonance spectrum supported the notion that ODC-2 may be indistinguishable from digoxin. The possibility that substances quite similar to cardenolides are synthesized in the mammalian body must be seriously considered.