Commonly, non-thermal plasma sources for medical applications are compared by the quantity of reactive components they produce. Here, a different perspective is chosen: A cell viability assay, which can act as standard procedure to compare plasma sources by the biological effectiveness is proposed. This assay can easily be performed with a minimum of technical equipment. It allows the estimation of the biological impact of a plasma source. For maximum flexibility, an indirect plasma treatment procedure is combined with a dilution method. The applicability of the assay is demonstrated by assessing the impact of different non-thermal plasma sources (kINPen & kINPen med, µAPPJ, water discharge, DBE) and the use of a series of human cell lines and their response to a kINPen treatment (HaCaT, MV3, Jurkat, THP-1, MRC-5).