We studied the potential for detecting oleander with a new immunoassay (Digoxin III, Abbott Laboratories, Abbott Park, IL) by comparing results with those from the fluorescence polarization immunoassay (FPIA) and Digoxin II assay (Abbott). In aliquots of drug-free serum pools supplemented with pure oleandrin or oleander extract, we observed apparent digoxin values using all 3 immunoassays, but values obtained by the Digoxin III were higher than obtained by the other assays. We also observed significant apparent digoxin values in vivo in serum samples of mice 1 and 2 hours after feeding oleander extract. The average half-life of digoxin-like factors was 1.1 hours. In a serum pool (prepared from patients taking digoxin) supplemented with oleander extract, the observed digoxin values were falsely lowered when measured by the Digoxin II but falsely elevated when measured by the Digoxin III and FPIA. Monitoring free digoxin using the Digoxin III cannot eliminate this interference. Digibind neutralized digoxin-like factors of oleander extract; the effect can be monitored by observing a significant reduction in apparent free digoxin levels in the presence of Digibind as measured in protein-free ultrafiltrate using the Digoxin III. The Digoxin III is highly sensitive for measuring oleander.