The objective of this study was to develop, optimize, and characterize a flexible and reliable unit for standardized and well-controlled electromagnetic-field exposure of cells in vitro. The technical requirements were high peak and time-averaged exposure of the cells with a minimal temperature rise for the cell cultures, flexible modulation schemes, high uniformity, and low variability of exposure, as well as support of blinded protocols. The developed setup is based on two R18 waveguides resonant at 1800 MHz and operated with a computer-controlled signal and monitoring unit. The cells can be exposed in 35-mm Petri dishes either cultivated as monolayers or in suspension. For cell monolayers, the system provides an efficiency for the specific absorption rate (SAR) of >50 (W/kg)/W input power, a nonuniformity of the SAR distribution of <30%, SAR variability of <6%, and a temperature rise of <0.03°C/(W/kg) average SAR. For cell suspensions and provided that the cells are not in the meniscus area, a SAR efficiency of >10 (W/kg)/W, nonuniformity of <40%, SAR variability of <17%, and a temperature rise of <0.13°C/(W/kg) is achieved. The numerical dosimetry for the field and temperature distributions within the Petri dishes was verified using E-field and temperature probes. The temperature analysis has shown that the possibility of localized "hot spots" can be excluded.