Abstract Plasma-digoxin levels were measured in 76 outpatients. At the higher dose-levels significant differences were found between the digoxin levels of patients using 'Lanoxin' tablets* and those using other brands. Maintenance plasma-levels and absorption curves of digoxin were studied in 7 patients both when taking lanoxin and when on another brand. Clinically significant differences in plasma-levels were seen in 2 of these patients, and in 1 the change was fourfold. The lower plasma-levels in these patients were due to decreased absorption of the drug. When the tablet material was crushed and given as a capsule, much higher plasma-levels were obtained. It is concluded that delayed tablet disintegration sometimes causes low biological availability of digoxin.