The percent of unbound lidocaine in the plasma of 24 healthy subjects was measured by equilibrium dialysis after addition of 3 microgram/ml C14 lidocaine hydrochloride. The percentage of unbound lidocaine varied from 19.9 to 38.8 (30.2 +/- 5, mean +/- SD) was inversely related to the concentration of alpha 1-acid glycoprotein (AAG) in the plasma (r = -0.931, p less than 0.001). The binding ratio (number of moles bound divided by number of moles unbound) of lidocaine was directly related to the plasma AAG concentration (r = 0.960, p less than 0.001). The binding ratio of lidocaine in solutions containing AAG but no albumin, prepared from the plasma of subjects in the study, was also directly related to the concentration of this acute-phase protein (r = 0.909, p less than 0.001). Human serum albumin solution (4 gm/100 ml) bound lidocaine to the extent of 20% under the same conditions. There was no relationship between the binding ratio of lidocaine and the albumin concentration in the plasma of the 24 subjects. In 7 normal subjects variation in AAG between 2 samples collected at least 1 mo apart was associated with a concomitant change in plasma lidocaine binding (r = 0.943, p less than 0.01). Thus even in normal subjects there is considerable interindividual and intraindividual variation in lidocaine binding, and measurements of AAG concentration in plasma may be a useful predictor of the extent of lidocaine plasma binding.