This is the first presentation of information on the circulation of human herpes virus type 6 (HHV-6) in the population of the mid-European part of this country. The results of the study indicate that HHV-6 is widely spread both among apparently normal people and among patients with different pathologies. The studies using immunofluorescence test showed that the percentage of HHV-6-infected subjects was practically similar among blood donors, patients with nasopharyngeal cancer, rheumatoid arthritis, hemophilia, and patients with transplanted kidney, varying from 34% to 49%. A significantly higher per cent (66%) and higher antibody titres to HHV-6 were detected in sera from patients with hemoblastoses. A high level of HHV-6 infection among patients with hemoblastoses was also confirmed by the results of immunoblotting showing a wider spectrum and higher intensity of the detectable proteins. It should be noted that the amounts of virus-specific proteins detectable by some sera showed individual variations indicating different levels of expression of viral proteins in different individuals.