[en] In this paper we present preliminary results of examination of the biological efficiency of the Petten Research Reactor mixed beam with respect to 250 kV X-rays for the induction of DNA damage and chromosomal aberrations in human lymphocytes. Human blood samples or isolated lymphocytes were irradiated by the beam from Research Reactor in ECN Petten, Netherlands and dose response relationships for the level of damage induced were investigated. In order to check any enhancement effect due to the process of boron neutron capture, chemical pretreatment with boric acid or mercaptoborane (containing boron-10 ions) was done. The estimation of the DNA damage was done with the use of a single cell gel-electrophoresis method (SCGE), to asses the frequency of chromosomal aberrations culturing of lymphocytes for the evaluation of cytogenetic damage was performed. Abnormal behavior of blood samples during a culture procedure and abnormally low metaphases frequency was noticed. During the analysis of DNA damage by SCGE assay we have also found the abnormalities in shapes and brightness of investigated comets. Part of the studied lymphocytes was bigger than others and had much bigger fraction of the DNA in tail. Very poor dose response relationship was observed in those results. From this reason, our paper presents the methodological approach and discussion of the results obtained and also studies on the parameters reflecting the level of the DNA in human lymphocytes. In order to eliminate outstanding comets (fluffy) we measured for all our results the relation of the fraction of DNA in tail to the length of the comet tail. The value of this ratio usually fluctuated in range of 0.1 to 0.70. For the fluffy comets mentioned before the tDNA/TL ratio was generally about 0.9, or even more than 1.0 that means that the percentage of fraction of DNA in tail was higher than in usually seen comets with such a tail length. After analysis of distribution of frequency cells with various tDNA/TL ratio, we decided to establish 0.8 as cut off value for the filtration software to eliminate the outstanding results. Elimination of the fluffy comets corrected our results and let observe various dose response relationships for various treatments (X-rays, modified beam of reactor neutrons in normal and boron pretreated cells). Applied correction usually leaded to improvement of statistics. Our additional studies showed that appearance of outstanding comets is also observed as results of disturbance of suspension medium. We have achieved the similar fluffy comets studying with application of the Comet assay the influence of hypotonic solution in cell suspension on comets shape after electrophoresis. (author)