Experiments performed to optimize the growth conditions of HHV-6(Z29) revealed that the virus grows best in phytohemagglutinin-stimulated umbilical cord blood lymphocytes (CBL) cultured in media containing 32 units/ml interleukin-2 and 0.01 mg/ml hydrocortisone. The titer of maternal antibody in the plasma of the cord blood cells does not affect the ability of the cells to support virus growth. DEAE-dextran and polybrene do not increase virus growth in umbilical cord blood lymphocytes. Phorbol myristate acetate abolishes virus growth. The HHV-6(Z29) growth cycle in CBL was approximately 5 days; capsids were not seen before day 3, and mature virions were not seen before day 5.