1. Pure erythrocytes preparations, free from platelets and white cells, were incubated for a long time without hemolysis. 2. Dose-response experiments performed with (a) cardioglycosides (ouabain and K-strophantoside), (b) steroid hormones and their glucuronides (tetrahydrocortisol, oestradiol and the respective 3-glucuronic derivatives) and (c) ouabain-like compounds purified in our laboratory (0.7 kDa and 2-4 kDa respectively) emphasise a modulatory effect [activation of Na+ efflux rate and K+ uptake at very low ligand concentrations, inhibition at higher levels; maximum enhancement of cation transport: (a) and (b) 10-0.1 nM (+40-50%), (c) 1-0.01 nM (2.5-fold)]. 3. Binding experiments show upward-curved Scatchard graphs, with the Kd values of 50 nM and 18 microM and the Bmax values of 10.2 and 984.5 fmol/100 microliters RBC (red blood cells) respectively.