Sera from 52 children with acute leukemia were examined for antibodies to herpes viruses (Epstein-Barr ( E B V ) ; herpes simplex ( H S V ) ; cytomegalovirus (CMV); varicella, and to some other common viruses (measles, rubella, mumps, adenovirus). The incidence of sera positive for herpes virus antibodies was lower among children 1-8 years of age with acute lymphoblastic leukemiaeither untreated, in relapse or in remissionthan among sera from age-matched children without leukemia. This difference was particularly striking among children less than 4 years of age, C M V and varicefla antibodies were not found in the sera of children 1-8 years of