Maximum levels of binding of ?-bungarotoxin to foetal human brain membranes were found to remain essentially constant at 30-50 fmol/mg protein (1.1-1.5 pmol/g wet weight in whole brain) between gestational ages of 10 and 24 weeks. Equilibrium binding of ?-bungarotoxin to both membranes and to detergent extracts showed saturable specific binding to a single class of sites with K(d) (app) values of 3.5 x 10(?9) M and 2.4 x 10(?9) M respectively. Association rate constants, determined from time courses of binding of ?-bungarotoxin to membranes and detergent extracts, were 2.3 x 10(5) M(?1) sec(?1) and 2.6 x 10(5) M(?1) sec(?1) respectively. Dissociation of ?-bungarotoxin from both membrane and detergent extracts showed a rapid initial rate with T (1 2 ) approx 15 min which, in the case of the detergent extract, was followed by a slower dissociation accounting for the remaining 20% of the bound ligand. Competition studies with a number of cholinergic ligands indicated that the ?-bungarotoxin-binding sites in foetal brain display a predominantly nicotinic profile.