Lymphocytes from eight patients with measles and six patients with varicella were studied during the acute phase (first week) of illness and after recovery for spontaneous and pokeweed mitogen (PWM)-induced production of immunoglobulins (Ig) and viral antibodies by an enzyme linked immunosorbent assay (ELISA). In both infections acute phase lymphocytes showed increased spontaneous in vitro IgM and IgG productions including IgM and IgG antibodies to the aetiological virus as well as IgG antibodies to unrelated viruses (varicella, measles, rubella and mumps) to which the patient had serum antibodies. PWM induced no further Ig synthesis in the acute phase. In the convalescent phase viral antibody production could be demonstrated only in PWM stimulated cultures. In four patients the spontaneous synthesis of antibodies to a non-aetiological virus seemed to precede the production of IgG antibodies to the aetiological virus. All patients showed an increase of ELISA determined serum antibodies to the aetiological virus from the acute to the convalescent phase. Three of seven measles patients also showed a minor but significant increase or decrease of serum IgG antibodies to varicella and one of six varicella patients a significant rise of serum IgG antibodies to measles. Thus both measles and varicella infections were associated with non-specific as well as specific B cell activation. The non-specific B cell activation may be induced by non-specific helper factors from activated T cells.