In this work, the investigation of the effects of 2,4,5-trichlorophenol (2,4,5-TCP), 4,6-dichloroguaiacol (4,6-DCG), and 4,5-dichlorocatechol (4,5-DCC) on selected morphological and biochemical parameters in human peripheral blood lymphocytes were studied. All of the investigated compounds (at concentrations from 25-600 ppm) increased the size and granularity of the lymphocytes. 2,4,5-TCP induced the strongest and 4,5-DCC induced the weakest changes in these parameters. Moreover, 2,4,5-TCP induced the greatest loss of lymphocyte viability, which was statistically significant at concentrations of 125 and 600 ppm. DNA and protein damage was provoked by relatively low concentrations of the xenobiotics examined. Comet assay analysis showed that 4,6-DCG and 4,5-DCC at 5 ppm significantly increased the level of single- and/or double-strand breaks in the DNA of human lymphocytes. The increase in carbonyl group content (the marker of protein damage) was more strongly induced by 4,5-DCC and 2,4,5-TCP than by 4,6-DCG at concentrations ranging from 0.04 to 5 ppm. DNA and protein damage was most probably caused by reactive oxygen species (ROS) because it was observed that all of the compounds studied, as well as 4,5-DCC and 2,4,5-TCP in particular, were capable of oxidising fluorescent probe 6-carboxy-2',7'-dichlorodihydrofluorescein at very low concentrations (0.01-1 ppm). In summary, 2,4,5-TCP induced the greatest morphological and cytotoxic changes in human peripheral blood lymphocytes, whereas its metabolite 4,5-DCC caused the most severe biochemical alterations, such as protein and DNA damage as well as ROS formation, in the incubated cells,.