Monolayers of small metal clusters of type Au55[P(C6H5)3]12Cl6 were investigated with a low-temperature ultrahigh vacuum scanning tunnelling microscope. Apart from the usual charge-quantization phenomena, such as Coulomb blockade and staircase, negative differential resistance was observed by performing measurements at distinct locations on the cluster layers. The latter phenomenon can be understood from a ‘gate’ effect caused by neighbouring clusters and involving a nonclassical behaviour of the capacitances generated by the nanoscale tunnel junctions.