An imaging agent for cells which produces an intracellular imaging signal proportional to the amount of hCE-1 in the cells independently of the amount of hCE-2 and/or hCE-3 in the cells, said imaging agent being a covalent conjugate of (a) an imaging agent and (b) an alpha mono- or di-substituted amino acid ester, wherein (a) is directly linked to (b), or (a) is indirectly linked to (b) by a linker radical, and wherein said direct or indirect linkage is via the amino group of (b), and wherein the amino group is not directly linked to a carbonyl group, and wherein the said alpha mono- or di-substituted amino acid ester part is selectively hydrolysable to the corresponding carboxylic acid part by the intracellular carboxylesterase enzyme hCE-1 relative to the intracellular enzymes hCE-2 or hCE-3.