The prevention of an artifact in receptor binding assay by an improved technique.

The intensity of vacuum suction used during filtration of cerebral membranes on glass fiber filters used in the assays of neurotransmitter receptors is a factor in determining the apparent extent of bound radioactivity. This is not related to protein loss during filtration at various speeds nor to rates of dissociation of various ligand-receptor complexes. It appears to be due to the failure of hydrophobic ligands on dry filters, to be readily removed by aqueous media. A means whereby the vacuum can be held constant is described and shown to eliminate this artifact.