We have isolated a labile, specific sodium pump inhibitor or digitalis-like factor from the peritoneal dialysate of volume-expanded renal failure patients whose levels correlated closely with volume status and blood pressure. This study characterizes the inhibitory profile of this agent compared with that of ouabain against the three alpha-isoforms of the sodium pump. We prepared microsomal Na,K-ATPase from rat tissues representing the highest proportion of one of the alpha-isoforms. Both Northern and Western blot analyses confirmed that kidney had predominantly the alpha1-isoform, skeletal muscle the alpha2-isoform, and fetal brain the alpha3-isoform. Ouabain (5 x 10-6mol/L) produced little inhibition of kidney Na,K-ATPase (3.4 +/- 2.0%) but significant inhibition of skeletal muscle (37.2 +/- 3.7%, P < .001) and fetal brain (38.8 +/- 3.5%, P < .001) activity. In contrast, the labile digitalis-like factor, causing comparable inhibition of fetal brain Na,K-ATPase activity (33.3 +/- 4.7%), produced markedly greater inhibition of kidney (42.5 +/- 5.6%, P < .001) and moderately greater inhibition of skeletal muscle pump activity (57.7 +/- 6.3%, P < .05). In addition, the labile digitalis-like factor produced a marked concentration-dependent inhibition of the alpha2- and alpha3-isoforms (r = .79, P = .00005). Experiments combining the labile digitalis-like factor and ouabain confirmed that digitalis-like factor, unlike ouabain, was an effective inhibitor of all three isoforms in rat, in particular alpha2. The different pattern of isoform sensitivity displayed by the labile digitalis-like factor and ouabain further differentiates the two agents and raises some interesting possibilities about the functional implications of the endogenous factor. (Hypertension. 1996;29:815-821.)