A group of patients with a history of recurrent tonsillitis were observed during an acute episode to determine the cause of the infection. The microbial pathogen that was consistently implicated was the Epstein-Barr virus. Seventeen (65%) of 26 patients exhibited a substantial seroconversion to the early antigen of Epstein-Barr-virus-infected lymphoblastoid cells (P3HR-1). We conclude that there is a high incidence of tonsillitis associated with the Epstein-Barr virus. The propensity of the virus for the palatine tonsils, a rich source of B cells, in suggested. Furthermore, the value of monitoring early antigen titers to confirm the nature of the infection is apparent, bearing relevance to future studies of this virus.