DNA sequences, antigens and elevated antibodies to HHV-6, and DNA sequences of HHV-7 in patients with Hodgkin's disease and non-Hodgkin's lymphoma have been detected. It is not known whether HHV-6 variants A and B, and HHV-7 contribute to the malignization by different ways, there is any interaction between these viruses, and their primary or recurrent infections occur during the disease progression. Total and high avidity IgG, IgM to HHV-6A, HHV-6B and HHV-7 were quantitated simultaneously in the sera of 12 patients with lymphomas and 12 control persons by indirect immunofluorescent assay and ELISA. It was established that, primary infection by HHV-6B in Hodgkin's disease, its primary or recurrent infections in non-Hodgkin's lymphoma; primary or recurrent infection by HHV-6A in Hodgkin's disease, its recurrent infection in non-Hodgkin's lymphoma; recurrent infection by HHV-7 in Hodgkin's disease may contribute to the deterioration of clinical conditions. Probably, HHV-7 exerts its effects through activating HHV-6B. The simultaneous effects of HHV-7 and HHV-6A, and that of HHV-6B and HHV-6A seem to be independent. Our results supports the recent opinion that, the effect of these herpesviruses on the tumorous cells is exerted indirectly by altered mediators of the immune system.