Increased use of mobile phones in the context of its implications on health issues is a serious concern. Here we investigated the effects of 2100 MHz W-CDMA (wideband code division multiple access) modulation radiofrequency (RF) field, the carrier of signals emitted by cellular phones, on free radical formation in cells. Human lymphocyte, leukemia (Chronic Lymphocytic Leukemia, MEC 1), breast fibroblast and breast adenocarcinoma (MCF-7) cell cultures were used in the experiments. The cell cultures were exposed to W-CDMA modulated RF radiation at 2100 MHz at a SAR level of 0.607 W/kg for 4 and 24 h. In addition, healthy lymphocytes and MCF-7 cells were exposed to Ultraviolet A (UV-A) radiation for 5 min equates to 26.85 J/cm(2) after 24 h RF exposure to stimulate reactive oxygen species (ROS) production as a positive control. Exposures to RF radiation were performed within a CO2 incubator. Radicals concentration was measured by spin trapping technique of Electron Spin Resonance (ESR) spectroscopy. We were not able to detect short-lived free radicals by ESR technique in RF exposed cells; however, we observed free radical ESR signals in the RF exposed cells after UV-A exposure.