The Authors have already related in various papers the results of their research on humoral and cellular immunity in the course of viral hepatitis in relation to the behaviour of HBsAg determined with various methods and recently with the radio-immune assay. Since the high sensitivity of this latter technique consents a sufficiently precise differentiation of hepatitis into HBAg-positive (hepatitis B) and HBAg-negative (hepatitis A), the Authors have intended studying in this work the eventual clinical epidemiological and immunological differences between the two types of hepatitis, analyzing the results of their previous studies, extending the case data and gathering numerous other personal observations, clinical and laboratory, not yet published. The study was limited to 136 patients with viral hepatitis in wwhich the test for HBsAg was effected with RIA: of these 58 were HBAg+ and 78 HBAg-. Initially is discussed the incidence of two types of hepatitis in relation to age and sex and then the comparative incidence between them of the various routes of contagion. Particularly emphasized is the possibility of direct transmission of HBAg+ hepatitis, often responsible for infection within the family. The comparative determination of transaminases, bilirubin, immunoglobulins and complement has revealed some interesting variations in the behaviour of the two types of hepatitis. Instead, cellular immunity, studied by the rosette E technique, appeared depressed in the initial phase of sickness in both types of hepatitis. Space is dedicated to the behaviour of HBsAg in acute HBAg+ hepatitis in relation to the immunoglobulins and complement and in relation to the normalization of the transaminases and bilirubinemia. After a few brief observations on the frequent appearance of a joint symptomatology in the course of viral hepatitis, particularly in the HBAg+ form, the Authors report the data relative the evolution of the hepatitis cases examined. The percentage of a complete cure are about equal between hepatitis A and hepatitis B even recovery is significantly faster in the first type of hepatitis (HBAg-).