Previously, we reported that various levels of acetylcholine (ACh), currently known as a neurotransmitter, are detectable in the blood of several mammals including humans and that most blood ACh originates from T-lymphocytes. To investigate whether ACh in the blood acts on lymphocytes and participates in the modulation of immune responses, we have analyzed the expression of mRNA for muscarinic (Ms) ACh receptor subtypes and nicotinic (Nc) ACh receptor subunits using reverse transcription-polymerase chain reaction (RT-PCR) methods. The cells tested were human peripheral mononuclear leukocytes (MNLs) from seven healthy donors and eight leukemic cell lines, as models of lymphocytes. We detected mRNA expression for various neuronal Nc receptor subunits and Ms receptor subtypes in all of the MNL samples and in all of the cell lines tested. However, the expression pattern of mRNA for neuronal Nc receptor subunits (alpha2-alpha7 and beta2-beta4) and Ms receptor subtypes (m1-m5) varied among the individuals and cell lines. No expression of mRNA for three muscle-type Nc receptor subunits (alpha1, beta1 and epsilon) was observed in the MNLs and cell lines. These results indicate that both neuronal-type Nc and Ms ACh receptors are present on the surface of lymphocytes.