Comparison of the incidence of abnormal variability in activity of glutamic-oxalacetic transaminase in the serum (S-GOT) in two groups of children who were household contacts of infectious hepatitis with the incidence in suitable control groups, demonstrated differences with a high degree of statistical significance. This difference was interpreted as evidence that a person with: a) a range of variation in S-GOT in excess of 22 Karmen units, b) exposure to a clinically recognized case of infectious hepatitis, and c) no other condition known to cause abnormality of S-GOT, may be considered to have infectious hepatitis himself, whether symptomatic or not. Immune globulin apparently prevented the abnormality in S-GOT expected in hepatitis contacts in one epidemic, but did not do so in another. Both in the presence and in the absence of jaundice, a significant percentage of patients with infectious hepatitis are characterized by persistent or recurring abnormalities in S-GOT for periods of at least several months. No correlation between magnitude and duration of abnormality in S-GOT and degree of clinical illness was observed. The ratio of icteric to non-icteric cases of infectious hepatitis was found to be a function of the age of the patient. The ratio was greater than one in the 10 to 14-year age group and less than one at other ages.