Free radical properties of different types of tumor cells were compared. Electron paramagnetic resonance (EPR) studies were performed for human (BM, IGR and SK) and mouse (B16 and S91) melanoma cells. In contrast to melanotic melanoma IGR, BM and B16 cells, amelanotic S91 cells contained only a trace amount of melanin. No EPR signals were detected for Caco2 cells and only a very weak EPR line was measured for fibroblast cells. Melanin does not exist in these cells. The aim of this work was the application of EPR spectroscopy to the determination of the kind of melanin (eu- or pheomelanin) in melanotic tumor cells. Microwave saturation of EPR spectra of tumor cells with high and low melanin content was compared. Eumelanin was identified in human BM, IGR, SK, and B16 melanoma cells. Single asymmetrical EPR lines were detected for these samples. The EPR spectra of human BM melanoma cells had the highest intensity. Paramagnetic centers in amelanotic S91 melanoma cells were also found. Trace amounts of eumelanin free radicals and the other free radicals in cells were responsible for their very weak EPR lines. The obtained results indicate that EPR spectroscopy is a very useful technique for the identification of melanin in tumor cells. Strong differences of microwave saturation of EPR lines for cells with high and low melanin content were observed. EPR lines of tumor cells with a low melanin content did not saturate at the used range of microwave power. Saturation was observed for melanotic BM melanoma cells.