An epidemiological survey of hepatitis B virus (HBV) infection has been carried out from 1974 up to 1978 in four laboratories, with the aim of identifying high-risk groups in the population. Radioimmunoassay procedures were used in order to detect the hepatitis B surface antigen (HBsAg) and the corresponding antibody (anti-HBs) in serum samples from over 100,000 individuals of various geographical origin (mainly from Ligury or from other areas in Northern Italy). The groups of population under monitoring included: individuals without hepatic illness just after admittance into hospitals (these groups were found to be adequately representative of the corresponding open population), groups of children and boys from the open school population, individuals living in various communities (foundling hospital, children college, recruits, institutionalized old people, subnormal individuals and their assistance staff), non-assistance working categories (workers from metallurgical and chemical industries, shipped seamen), hospital assistance personnel, dialyzed and transplanted renal patients, blood donors. The results obtained showed a considerable variability of HBV infections among the examined groups of population. In particular, the frequency of HBsAg and/or anti-HBs detection appeared to be significantly affected by each of the following epidemiological parameters: geographical area, sex, age, pregnancy, life in communities and contacts with hepatitis patients, mental deficiency, non-assistance working activities, assistance activity in the hospital environment, hemodialysis and transplants, selection of population groups by HBsAg screening.