Abstract: A device for investigating the effect of biologically active substances on microorganisms comprises a vessel having a bottom on which there is arranged a gel layer, and from the bottom of which vessel there extend upstanding walls which delimit a plurality of mutually adjacent, elongate, rectangular compartments. To facilitate the distribution of microorganisms of the gel layer, the compartments are open at one short end thereof in a manner such that the rectangular gel fields located in respective compartments merge directly at the open end thereof with a further gel field extending along the short side of all said compartments.