The level of 21 plasma proteins was followed in Hepatitis A for two months after onset of icterus. The mean concentration of alpha 1-antitrypsin, orosomucoid, haptoglobin, C-reactive protein (CRP) and alpha 1-antichymotrypsin increased uniformely during the first week of hepatitis A. Thus, they differ from that of inoculation hepatitis earlier described. The mean curve for IgM was higher in hepatitis A than the corresponding results for inoculation hepatitis during the first week of illness, but because of great inter-individual differences in concentrations IgM determinations can not be used to discriminate between the two diseases in a given case. IgA levels were slightly increased early in hepatitis A but no change in IgG levels was observed. Prealbumin was the best mirror of the patients' recovery or deterioration.