The invention relates to an apparatus and an in-situ method for detecting cellular DNA damage. The invention is particularly suited for use in Comet assays and for automation of such assays. The apparatus comprises a multiwell plate, the plate comprising an array of wells held in fixed relationship with each other and each well having an axis, side walls, and a base and wherein the well is provided with electrode pairs disposed therein; and wherein there is provided means for parallel connection of the electrode pairs to an external voltage supply.