We have characterised the muscarinic receptor subtypes found in human skin fibroblasts and compared binding levels in cell lines from members of the Alzheimer's disease family with the Swedish amyloid precursor protein (APP) 670/671 mutation. Binding studies with [3H] quinuclidinyl benzilate ([3H]QNB) and the M2/M4 selective antagonist [3H] (+/-)-5,11-dihydro-11-([(2-[(di-propylamino)methyl]-1- piperidinyl]ethyl)amino]carbonyl)-6H-pyrido(2,3-b)(1,4)benzodiazepine-6- one ([3H]AF-DX 384) revealed the presence of a single population of muscarinic receptors on lysed fibroblast membranes. [3H]QNB binding was displaced by a number of selective muscarinic ligands with a rank order of potency: atropine > himbacine > methoctramine > (+/-)-p-fluoro-hexahydro-sila-difenidol hydrochloride > pirenzepine > muscarinic-toxin-3. APP 670/671 mutation carrying cell lines showed 25-35% lower levels of muscarinic receptors labelled with [3H]QNB, [3H]N-methyl scopolamine and [3H]AF-DX 384, compared to controls. This difference was not statistically significant due to large individual variation. It is concluded that muscarinic receptors on adult skin fibroblasts are predominantly of the M2 subtype. Since these cells do not possess M1 and M3 receptor subtypes, they are unlikely to provide a good model for studying muscarinic receptor regulation of APP processing.