Poisoning from the oleander plant is common. Taking advantage of the high cross-reactivity of oleandrin, the major cardiac glycoside found in the oleander plant, we demonstrated that the serum digitoxin assay can be successfully used for the rapid diagnosis of oleander poisoning. Digitoxin is rarely used for treatment of cardiac disorders in the United States and has a therapeutic range of 19.7 to 39.3 nmol/L. In a typical oleander poisoning, serum oleandrin concentrations may reach 174 mmol/L or more. A serum specimen supplemented with 174 mmol/L of oleandrin containing no digitoxin showed an apparent digitoxin concentration of 1,272.1 nmol/L, a very high value compared with the range of the serum digitoxin assay, which is 2.6 to 104.8 nmol/L. Moreover, the response of the serum digitoxin assay with serum specimens containing various concentrations of oleandrin (and no digitoxin) is linear. Therefore, the oleandrin concentration in serum can be calculated from the apparent digitoxin concentration to access the severity of poisoning. Recently, the usefulness of the digoxin-specific Fab antibody fragment in the treatment of oleander poisoning has been described; however, no laboratory test was performed to demonstrate the progress of therapy. We demonstrated that the digoxin-specific Fab antibody can bind oleandrin in vitro, thus reducing the pharmacologically active free oleandrin. Because Fab and oleandrin bound to Fab are absent in the protein-free ultrafiltrates, monitoring the activity of free oleandrin in the ultrafiltrates can be used for monitoring the effectiveness of therapy.