Peroxisome proliferator-activated receptors (PPARs) play an important role in numerous chronic diseases such as diabetes, obesity, atherosclerosis and cancer, and PPAR modulators are among the approved drugs and drug-candidates for their treatment. The aim of this study was to elucidate the involvement of PPARs in the mechanism of cytotoxic and pro-apoptotic action of novel anticancer 4-thiazolidinone derivatives (Les-2194, Les-3377, Les-3640) and approved 4-thiazolidinones (Rosiglitazone, Pioglitazone) towards the human squamous carcinoma (SCC-15) cell line. Experiments with 4-thiazaolidinone derivatives and PPAR-specific siRNA were conducted and PPARalpha, PPARbeta and PPARgamma mRNA expression was studied. Moreover, after PPARalpha, PPARbeta and PPARgamma siRNA gene silencing, cell viability, cell metabolism and caspase-3 activity were measured. The results showed a decrease of mRNA expression of the studied PPARs in SCC-15 cells treated with 10 and 50 muM Les-2194, Les-3377 and Les-3640. PPARgamma knockdown protected the cells from the cytotoxic effect of the tested compounds (50 muM). It was established that novel anticancer 4-thiazolidinone derivatives act mainly through the PPARgamma pathway in SCC-15 cells. Our results suggest that all studied compounds act as PPARs agonists. Interestingly, silencing of PPARgamma gene increases the expression of PPARalpha, PPARbeta mRNA in SCC-15 cells. The anticancer potential of new studied compounds was more expressed as compared to Rosiglitazone and Pioglitazone.