A rapid but precise radioassay for digoxin has been developed by optimizing the conditions for antigen-antibody interaction and for separation of bound and free fractions. This new procedure involves incubation of radioactively labeled antigen, antibody, and standard or sample at 3 degrees C for 10 min. Charcoal is then added, the mixture centrifuged for 5 min, and the radioactivity of the supernate counted. Results are available in less than 1 h after receipt of the specimens, making the procedure suitable for emergency determinations. Results correlate well with those by our routine procedure (Becton Dickinson) (r = 0.9739, y = 0.0356 + 0.9915x, n = 49). Analytical recovery of added pure digoxin exceeds 94%, both within and between assay CV's are less than 8%.