The polymerase chain reaction was used to screen human peripheral blood mononuclear cells (PBMC) and Jurkat cells for the presence of GABA(A) receptor subunit mRNAs. Positive signals were detected for the alpha 1, alpha 3, beta 2, beta 3, delta and epsilon subunit mRNAs in both cell populations, with the Jurkat cells giving a positive signal for some additional species. Real-time PCR was used to confirm that PBMC, lymphocytes and monocytes contained significant levels of the alpha 1 subunit mRNA and that PBMC and lymphocytes contained low levels of beta 2 mRNA. The alpha 1 subunit was detected in PBMC and fractionated T-cell populations, as well as Jurkat and HL-60 cell lines, by Western blotting and immunofluorescence using a specific antibody. The application of 1 mM GABA reduced the specific increase in intracellular PBMC Ca2+ levels produced by addition of 1 nM fMLP: this effect was mimicked by muscimol, but not glycine, and was blocked by bicuculline. The inhibitory effect of GABA was limited to a subset of PBMC. We conclude that cells within the human PBMC population, including lymphocytes, express functional GABA(A) receptors and these receptors may modulate immune responses. (c) 2005 Elsevier Ltd. All rights reserved.