A fundamental study was performed to assess the binding characteristics of an ovine anti-digoxin-Fab preparations, DIGIBIND, for digoxin, using equilibrium dialysis. The average mass of the antibody fragment was determined by matrix-assisted laser-desorption mass spectrometry to be 46,200 Da. The main conclusions are that the binding affinity and capacity are similar for several batches, produced over a period of several years. The average median affinity for digoxin of five batches determined by Sips analysis was 3.4 x 10(11) M-1 (range 1.95-6.15 x 10(11) M-1) (corresponding to dissociation constants, Kd, in the range 1.63-5.13 pM), and the index of heterogeneity was 0.78 +/- 0.03.