Cord blood immunoglobulin M was measured in 3474 consecutive newborn infants. A group of 147 infants with elevated IgM values (>/=19.0 mg./100 ml.) were compared with 92 unselected newborn infants with normal IgM values. One infant with clinically unsuspected congenital rubella was detected in the study group while no cases of intra-uterine infection were found among the controls. A greater proportion of mothers in the study group had a history of viral infection. The study group also contained a larger number of mothers who might be considered to be at greater risk of infection with agents known to cause intra-uterine disease. Follow-up studies at 6 months of age revealed no differences between the two groups aside from an increased incidence of minor motor abnormalities in the study group. While it is recognized that infants with cord blood IgM levels truly in excess of 30 mg./100 ml. may represent a high-risk group with respect to proved or subclinical intra-uterine infection, it is concluded that routine cord blood screening for elevated IgM values is not a high-yield procedure for the detection of intra-uterine infection in our population.