Ligand bound to detergent-solubilized or cytosolic receptors can be separated from free ligand by filtration through glass-fiber filters which have been pretreated with polyethylenimine (PEI). Receptors which can be assayed by this technique include detergent-solubilized muscarinic, adenosine A₁, α₁-adrenergic, α₂-adrenergic, β-adrenergic, dopamine D₂, opiate, bradykinin, and benzodiazepine receptors as well as naturally soluble estradiol receptors. For muscarinic, adenosine, α₂, dopamine, and estradiol receptors, specific binding measured by the PEI-filter technique was 84–110% of specific binding measured by gel filtration, demonstrating that the technique gave almost quantitative recovery of bound ligand.