The sigma binding site present in the Jurkat human T lymphocyte cell line was investigated. Jurkat cell membranes were found to have a single saturable binding site for [3H]haloperidol, a sigma ligand (dissociation constant, 3.9 +/- 0.3 nM). The binding of [3H]haloperidol was inhibited by several sigma ligands. Northern analysis and reverse transcription-polymerase chain reaction provided evidence for the expression of the recently cloned type 1 sigma-receptor (sigma-R1) in Jurkat cells. The sigma-R1 cDNA cloned from these cells was functional in heterologous expression systems. When expressed in mammalian cells, the cDNA-induced binding was saturable with dissociation constants of 1.9 +/- 0.3 nM for [3H]haloperidol and 12 +/- 2 nM for (+)-pentazocine. The binding of [3H]progesterone, a putative endogenous ligand to sigma-R1, to the Jurkat cell sigma-receptor could be directly demonstrated by using heterologously expressed sigma-R1 cDNA. The binding of [3H]progesterone was saturable, with a dissociation constant of 88 +/- 7 nM. Progesterone and haloperidol interacted with the receptor competitively. Reverse transcription-polymerase chain reaction also produced evidence for the existence of an alternatively spliced sigma-R1 variant in Jurkat cells. This splice variant was found to be nonfunctional in ligand binding assays. This constitutes the first report on the molecular characterization of the sigma-receptor in immune cells.