The binding of [3H]physostigmine to crystallized human serum albumin (HSA) has been investigated using equilibrium dialysis. The percentage bound to 1% (w/v) HSA decreased from 18 to 4% as the total concentration of physostigmine increased from 3.3 nM to 2.7 microM (0.9 to 750 ng mL-1). A single class of specific binding sites with a large affinity constant, K = 8 x 10(7) L mol-1, was identified. The concentration of binding sites was approximately 3 nM. The Michaelis constants for human serum cholinesterase and albumin were the same; an explanation for these results is that the drug is binding to a trace cholinesterase, in the albumin.