Background: The exposure of cell lines to low-dose irradiation leads to changes at molecular level, which may induce adaptive response. We examined radio-adaptive doses effect on human colorectal adenocarcinoma cell line (HT29) and human fetal fibroblasts (MRC5) cell line followed by hyper and hypo fractionation regimes, with main purpose to decrease cell viability in HT29, and at the same time to spare MRC5 cells. Material and Methods: The cell lines were pre-irradiated with 0.03Gy, 0.05Gy and 0.07Gy. Two hours later, control and pre-irradiated cells were irradiated in hyper and hypo fractionation regimes. Cell viability and the total cell number were measured. Results: Comparing the response between two cell lines in the same regime, it was found that pre-irradiation dose of 0.05Gy increased cell viability in MRC5 cell line, accompanied with decrease of cell viability in HT29 cell line, which gave a major contribution to the main goal of the present research, i.e. to determine the dose that might spare the normal tissue. Conclusion: To our best knowledge, fractionation in several consecutive days in two designed regimes is described for the first time. These are the first reported results using low-doses pre-irradiation followed by hyper and hypo fractionation regimes, with approximately same biological effective dose.