The present study describes the T lymphocytosis in infectious mononucleosis (IM), using rosette techniques, lymphocyte transformation tests, Con A suppressor tests, and chromosomal analysis. Blood was drawn from twelve patients with IM during acute illness and during convalescence. We found a T lymphocytosis with a normal sheep erythrocyte receptor affinity of T cells, using three different E-rosette techniques. The percentage of T cells with Fc receptors for IgM (T mu) was reduced during acute illness, but normal in convalescence. The percentage of T cells with Fc receptors for IgC (T gamma) was normal during illness and slightly reduced afterwards. Owing to the T lymphocytosis the total number of T gamma lymphocytes was significantly increased during IM. The lymphocyte reactivity in vitro after mitogen stimulation (PHA, Con A, PWM) was in the lower end of the normal range. We found no evidence of increased suppressor-cell activity activity during IM, using a Con A suppressor cell assay. No chromosomaL defects were observed in lymphocytes from blood.