Botulinum neurotoxin, purified to homogeneity from Clostridium botulinum (Type A), was found to be highly neurotoxic (greater than 8 X 10(7) mouse LD50/mg protein). Labelling of this pure neurotoxin with 125I-iodine to high specific radioactivity was achieved without appreciable loss of biological activity. This was used to demonstrate saturable binding sites for this toxin at the neuromuscular junction, following in vivo administration into mice. A demonstrable inhibitory effect of the neurotoxin on release of acetylcholine from rat cerebrocortical synaptosomes indicates that it affects synapses in the central nervous system. Kinetic studies on the binding of 125I-labelled neurotoxin to brain synaptosomes yielded an association rate constant of 2.3 x 10(5)M-1s-1; dissociation plots were biphasic and the predominant species showed a rate constant of 1.2 X 10(-4)s-1. The saturable binding component is heat-sensitive and inactivated by trypsin. Preliminary studies showed that botulinum neurotoxin associates with plasma membrane fractions of synaptosomes and that binding does not result in any gross structural changes, at least in the majority of the toxin molecules.