A microplate reader and method has at least one measuring device and a holding device for accommodating at least one microplate and for positioning samples-containing wells of microplates in relation to the measuring device. The at least one measuring device is used for detecting light emitted by samples in wells of a microplate and/or which is influenced by samples transilluminated by light in the wells. The microplate reader has a control unit for controlling the composition of a gas atmosphere surrounding the wells containing the samples. A respective use is characterized particularly in that living cells are measured in a controlled gas atmosphere, wherein the living cells are chosen from microaerophilic, optionally anaerobic and obligatorily anaerobic microorganisms as well as fungi and eukaryotic cells.