The cholinergic system takes part in the immune-neuroendocrine integration. Lymphocytes have been found to express muscarinic and nicotinic cholinergic receptors, as well as acetylcholine-esterase on their surface, and cholinergic agents modulate immune functions in vitro and in vivo. In the present study we provide evidence that purified organ resident and circulating lymphocytes, as well as various lymphoid cell lines derived from different species, exhibit choline-acetyltransferase activity and, therefore, have the potential to produce the neurotransmitter acetylcholine.