This paper presents results of combined treatments of human lymphocytes by deltamethrin and X-ray radiation. Cells were stimulated and incubated for twenty-four hours to ensure their growth. To the cultures selected for chemical pretreatment deltamethrin was added. After four hours the medium in all cultures had been replaced and cells were incubated for another 20 hours. Then, selected samples were irradiated with acute (1 Gy) and split (twice 0.5 Gy) doses of radiation. Doses split into two parts had two hour long intervals between irradiations. Results of radiation performed alone showed a very low frequencies of chromosome type aberrations (dicentrics and rings) and elevated frequency of fragments, triradials and quadriradials instead. The results also showed a higher level of induced chromosome damage after a split dose than after the acute one. The frequencies of aberrations observed after irradiation of chemically pretreated cells were lower than values expected on the base of additive effect from individual treatments. The most visible protective effects of chemical pretreatment were observed in cases of acentric aberrations. It was visible also in number of aberrant cells induced by combined treatment of deltamethrin and fractioned irradiation. Differences between the levels expected and observed are statistically significant in all the cytogenetic endpoints measured but dicentrics and rings. Our results display that deltamethrin pretreatment is capable to induce also in mammal cells the resistance to subsequent damage induced by radiation.