The authors report the clinical results and the cost/benefit analysis of a protocol of passive prophylaxis of hepatitis B based on the administration of two doses of three ml each of HBIG and on the control of anti-HBs levels by RIA for six months after accidental exposure in health-care personnel. Only two out of 351 susceptible subjects developed viral hepatitis instead of the expected 70 if passive prophylaxis had not been performed. The cost/benefit analysis showed a saving of 356 509 US$ in the past 36 months. Furthermore, the cost of each case of hepatitis prevented was 2194 US$, whereas the cost of one case of acute, benign hepatitis was 6180 US$. Since a saving of 3.39 US$ for each dollar spent is achieved, this protocol of passive prophylaxis appears more than adequate both clinically and economically. It is believed that the adoption of some general prophylactic measures will further stress the advantages of this procedure.