HHV-7 growth on Sup-T1, an immature T-cell line, was studied using different HHV-7 isolates obtained in our laboratory. Titration of viral yields showed that all the virus isolates propagate on this cell line more efficiently than in cord blood lymphocytes, the cells usually recommended for HHV-7 growth. The permissivity of Sup-T1 to HHV-6, whose ability to replicate in these cells was still unknown, was also investigated using two virus isolates representative of variants A and B respectively. Both isolates were able to propagate on Sup-T1 and viral titres were similar to those obtained in cord blood lymphocytes. As the efficient propagation of both HHV-7 and HHV-6 isolates in Sup-T1 cultures, these cells may replace more time consuming and expensive cord blood lymphocyte preparations for the propagation of both the viruses.