Exclusion or confirmation of congenital infection can be difficult in newborn infants. The presence of an infective organism in a fetus leads to activation of fetal T lymphocytes. We have examined expression of isoforms of a T-cell surface molecule, CD45RO, in 119 infants, 8 of whom had an intrauterine infection diagnosed antenatally. CD45RO was expressed on fewer than 10% of T cells in control infants and on more than 17% of T cells from infants with known infection (p < 0.006). This method allows screening of infants for infection by a range of organisms as a cause of their infection.