ASI-222,3beta-O-(4-amino-4,6-dideoxy-beta-d-galactopyranosyl) digitoxigenin, is a semi-synthetic cardiac glycoside patterned after a natural glycoside obtained from Cambodia. Effects of ASI-222 on contractile force in the isolated rabbit atria, cardiac contractile force, cardiac rate, ventricular excitability and functional refractory period in dogs, and acute toxicity in mice have been compared to those effects of ouabain. Both electrically driven and spontaneously beating atria demonstrated more rapid onset and greater maximum increases in contractile force with ASI-222 than with ouabain in equal bath concentrations. In the dog, ASI-222 increased cardiac contractile force more rapidly and at a lower cumulative dose than ouabain. Moreover, the maximum increase in contractile force obtained with ASI-222 was greater than that obtained with ouabain. The occurrence of ventricular ectopic beats was observed at a higher cumulative dose of ASI-222 than for ouabain. Also, ASI-222 produced a decrease in ventricular excitability and an increase in functional refractory period ot the ventricle. Ouabain, in the same molar dose, produced either no change or a slight increase in these parameters. Our data indicate that ASI-222 has a greater therapeutic index than ouabain. This difference may be partially explained by effects of ASI-222 on electrical properties of the heart.