Measurement of an organism's, organ's, tissue's or cell's capacity to respond, such as by a change in activity, state or condition, to acetylcholine (ACh), the ester of acetic acid and choline with chemical formula CH3C=OO(CH2)2-N(+)-(CH3)3 and IUPAC name 2-acetoxy-N,N,N-trimethylethanaminium, which acts as a neurotransmitter.