Abstract Immunoglobulins G, A and M were determined during the course of disease in 19 cases of sporadic infectious hepatitis, eight cases of epidemic infectious hepatitis, 11 cases of serum hepatitis, five cases of cholestatic jaundice, and 15 cases of obstructive jaundice. A marked increase was found initially in both infectious hepatitis groups (367% ± 160 and 376% ± 145, respectively), which fell precipitously during the first weeks of illness. Significantly lower γM levels were found in the serum hepatitis group (210% ± 82), and five of the cases showed normal immunoglobulin levels. γG levels rose to a lesser extent (158% ± 54 and 142% ± 36) in the infectious hepatitis patients but tended to remain elevated longer. There was no significant difference between infectious hepatitis and serum hepatitis (131% ± 37). Small but significant increases in γA levels were observed in infectious hepatitis. Slight or no changes occurred in γD levels in three cases of infectious hepatitis and two cases of serum hepatitis. No alterations of immunoglobulin levels were observed in connection with the onset of cholestatic or obstructive jaundice, indicating that liver damage per se does not give rise to the changes observed in viral hepatitis. A close correlation was observed in this material of previously healthy individuals between thymol turbidity and γM (r=0.76).