The effectiveness of neutrons in relation to X-rays in the induction of various abnormalities in the Tradescantia stamen hair cells of two clones heterozygous for the flower color were assessed. Dose response relationships for various endpoints (gene and lethal mutations and cell cycle alterations) in somatic cells of Tradescantia plants clone 4430 and clone 02, were established for X-rays and 5.6 MeV neutrons from U-120 cyclotron in Cracow. The radiation absorbed doses were 0.07-2.5 Gy in case of neutrons and 0.1-5.0 Gy in case of X-rays. These data were used to calculate the relative biological effectiveness (RBE) for 5.6 MeV average energy neutrons. RBE values were in the range of 3-9, depending on the endpoints. Dependence of mutation rates on the postexposure time-delays of culture also were studied and it was shown that it had no significant effect on RBE values estimated on the base of Tradescantia results. Presented data indicate that Tradescantia is an appropriate model system for RBE determination at low doses of radiation.