[en] A kappa opioid binding site has been characterized on the macrophage cell line, P388d₁, using the kappa selective affinity ligand, [^3H(1S,2S)-(-)-trans-2-isothiocyanato-N-methyl-N-[2-(1-phrrolidinyl) cyclohexyl] benzeneacetamide ((-)BD166). The kappa site has a relative molecular mass (Mr) of 38,000 under nonreducing conditions and 42,000 under reducing conditions. Moreover, it exhibits enantioselectivity in that 1S,2S-(-)-trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide ((-)-U-50,488) blocks [³H]95α,7α,8β)-(-)-N-methyl-N-[7-(1- pyrrolidinyl)-1-oxaspiro-(4,5)-dec-8-yl]benzeneacetamide (U-69,593) binding to P388d₁ cells with an IC₅₀ = 7.0 nM whereas 1R,2R-(+)-trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide ((+)U-50,488) blocks [³H]U-69,593 binding to P388d₁ cells with an IC₅₀ = 700 nM