A radioimmunological assay was made of HBsAg and HBsAb in 183 patients during the course of acute viral hepatitis, 23 with prior HBsAg-positive hepatitis, 72 with chronic liver disease, 822 blood donors, 44 patients and 28 staff members from two dialysis centres, and the medical and paramedical staff of several high-risk departments. Microimmunodiffusion on agar was also used to determine HBeAg and HBeAb in the same 183 acute hepatitis patients, 37 HBsAg+ blood donors, 20 asymptomatic HBsAg carriers, the patients of two haemodialysis centres and the staff of departments at high risk for hepatitis B. Attention is drawn to the marked incidence (5.5%) of chronic HBsAg carriers in the blood donors, and the considerable circulation of virus B in the population examined, HBsAb-positivity (27.6%) being also an expression of this. Stress is also laid on the diagnostic and prognostic significance of the study of the HBsAg-HBsAb system during acute and chronic hepatitis. The importance of serum hepatitis B markers in the transmission of this disease in dialysis centres and other high-risk departments and in its prevention is underscored. Lastly, emphasis is laid on the appreciable progress that analysis of the e-anti-e system offers in the prognostic assessment of type B hepatitis, attention being also drawn to the fact that the dynamics of this system is even more complex and interesting than had first been thought. An assessment of this kind is useful in the differentiation of "contagious" and "non-contagious" HBsAg carriers.