During the course of acute viral hepatitis A and B, several humoral and cellular immune phenomena have been observed, part of which is predominantly or even exclusively associated with hepatitis B: 1. Relative and absolute counts for T-lymphocytes depressed and for "null" -cells elevated; 2. mild elevation of serum globulin levels; 3. IgM augmentation occurring fastly, pronounced, and long lasting in typical cases; 4. IgG augmentation occurring later, less pronounced, and for a shorter period in typical cases; 5. autoantibodies to smooth muscles and mitochondria in low titers in some patients; 6. specific antibodies to "e" -antigen (early) and HB-Ag (later in the course) in part of the cases with hepatitis B; 7. immune complexes including HB-Ag, IgG and probably IgM (and IgA) as well as complement in some cases; 8. depressed levels of the fourth component of complement and - in cases complicated by "allergic" symptoms - of C3, C4, and total complement; 9. occurrence of activated lymphocytes ("virocytes") in peripheral blood; 10. enhanced spontaneous lymphocytic DNA-synthesis; 11. enchanced phytohaemagglutinin stimulation of lymphocytes; 12. mild lymphocyte proliferation to HB-Ag in part of the acute and convalescent cases of hepatitis B; 13. production of migration inhibition factor to liver specific protein (and HB-Ag) or lymphocytes in different percentages of patients with hepatitis B. Origin, diagnostic and prognostic importance, as well as pathogenetic revelance of the described immune phenomena are discussed.