Abstract Acetylcholine (ACh) is a well characterized neurotransmitter occurring throughout the animal kingdom. In addition, both muscarinic and nicotinic ACh receptors have been identified on lymphocytes of various origin, and their stimulation by muscarinic or nicotinic agonists elicits a variety of functional and biochemical effects. It was thus initially postulated that the parasympathetic nervous system may play a role in modulating immune system function. However, ACh in the blood has now been localized to lymphocytes; indeed expression of choline acetyltransferase (ChAT), an ACh synthesizing enzyme, has been shown in human blood mononuclear leukocytes, human leukemic T-cell lines and rat lymphocytes. Stimulation of T-lymphocytes with phytohemagglutinin activates the lymphoid cholinergic system, as evidenced by increased synthesis and release of ACh and increased expression of mRNAs encoding ChAT and ACh receptors. The observation that M3 muscarinic receptor stimulation by ACh and other agonists increases the intracellular free Ca2+ concentration and upregulates c-fos gene expression strongly argues that ACh, synthesized and released from T-lymphocytes, acts as an autocrine and /or paracrine factor regulating immune function. These findings present a compelling picture in which immune function is, at least in part, under the control of an independent lymphoid cholinergic system.