Abstract: The presence of a higher concentration of digoxin-like immunoreactive substances(DLIS) in human serum have been reported in a number of pathophysiological conditions. DLIS can react with antidigoxin antibodies when determination of serum digoxin was performed with immunoassay. It may falsely elevate the serum digoxin concentration and is troublesome in the therapeutic monitoring of digoxin. The apparent digoxin concentrations(DLIS) in serum were determined in 15 elder patients with cardiac insufficiency by fluorescence polarization immunoassay(FPIA). The lowest measurable concentration, defined as the concentration that could be distinguished from zero with 95% confidence, was 0.256 nmol·L^-1. Steroids and drugs commonly administered with digoxin showed no significant cross-reactivity. In these patients, the positive ratio for determination of DLIS was 46.7%(7 in 15 cases), and its mean serum concentration was 0.55±0.44 nmol·L^-1 (range 0.26～1.52 nmol·L^-1). The results were well consistent with that obtained by Dasgupta et al. In conclusion, there are certain increased concentrations of DLIS in the sera of elder patients with cardiac insufficiency, suggesting that digoxin levels measured by immunoassay method must be interpreted carefully in these patients after treatment with digoxin or other digitalis preparations.