The present study investigated the effect of plasma-produced reactive oxygen (ROS) and nitrogen (RNS) species on cancer cell viability. Reactive species were generated in deionized water by using an atmospheric pressure Ar plasma jet within the controlled ambient gases (air, O 2 or N 2 ), which allowed the production of plasma-activated water containing only ROS (e.g. O 3 , H 2 O 2 ) or both ROS and RNS (e.g. H 2 O 2 , NO 2 – , NO 3 – ). A considerable amount of H 2 O 2 was produced in all ambient gases, and its generation rate was highest in N 2 and lowest in O 2 . The latter was connected with a H 2 O 2 precursor, OH, efficient quenching in O 2 ambient gas. Small quantities of NO 2 – were generated during short (< 5 min) plasma treatments in ambient air and N 2 . The highest amount of NO 3 – was produced in N 2 ambient gas. Ozone was detected only in the case of O 2 environment. Cell viability studies were carried out by utilizing two cancer cell lines: 4T1 (breast cancer) and PPC-1 (prostate cancer). The results of the colorimetric succinate dehydrogenase activity assay showed that the studied cell lines had a similar sensitivity to the plasma activated medium. The impact of medium produced in the O 2 ambient environment was determined by H 2 O 2 content. The equivalent amount of H 2 O 2 in the plasma activated medium produced in the N 2 ambient environment caused an almost two-fold higher viability than in the case of the O 2 ambient gas. It is proposed that this was due to the cellular proliferation enhancing effect of NH 3 .