The presence of acetylcholinesterase (AChE) mRNA and activity in the tissues and cells involved in immune responses prompted us to investigate the level and pattern of AChE components in spleen. AChE activity was higher in mouse spleen (0.46 +/- 0.13 micromol of acetylthiocholine split per hour and per mg protein) than in muscle or heart, but lower than in brain. The spleen was essentially free of butyrylcholinesterase (BuChE) activity. About 40% of spleen AChE was extracted with a saline buffer, and a further 40% with 1% Triton X-100. Sedimentation analyses, the splitting of subunits in AChE dimers, phosphatidylinositol-specific phospholipase C (PIPLC) exposure, and phenyl-agarose chromatography showed that hydrophilic (G1H, 43%) and amphiphilic AChE monomers (G1A, 36%), as well as amphiphilic dimers (G2A, 21%), occurred in spleen. All these molecules bound to fasciculin-2-Sepharose, although the extent of binding was higher for G1H (77%) than for G1A (63%) or G2A (48%) forms. Differences in the extent to which wheat germ lectin (WGA) adsorbed with AChE of mouse spleen and of erythrocyte allowed us to discard the blood origin of spleen AChE activity. A 62 kDa protein was labeled in spleen samples using antibodies against human AChE. The protein was attributed to AChE monomers since its size was the same, regardless of whether disulfide bonds were reduced or not. Since cholinergic stimulation modulates proliferation/maturation of lymphoid cells, AChE may be important for regulating the level of acetylcholine (ACh) in the neighborhood of cholinergic receptors (AChR) in spleen and other lymphoid tissues.