Three siblings developed severe (two) or fatal (one) infectious mononucleosis. This family differed from previously described kindreds with a susceptibility to overwhelming Epstein-Barr virus infections in that: (1) both males and females were affected: (2) they had a history of recurrent bacterial infections; (3) they produced the full spectrum of antibodies to EBV in the expected range of titers; and (4) survivors recovered completely. Two of these youths, but not their parents or an unaffected sibling with mild IM, had a deficiency of natural killer activity that did not respond to preincubation of their peripheral blood mononuclear cells with interferon. NK activity may have an important role in controlling infections with EBV.^***