In several countries where there is a high prevalence of HBV the risk of PTH has been investigated. One study demonstrated that HBV immunity provided protection against infection following transfusion of HBS Ag-positive blood in that a higher proportion of immune recipients failed to develop, or developed only transitory, HBS antigenemia. Forty (61 per cent) of 66 recipients who received HBS Ag positive (IAHA) units developed hepatitis, which was subclinical in 37 (93 per cent) of the 40. Six recipients of HBS Ag-containing units had HBS Ag in the pre-transfusion serum. In four, HBS Ag disappeared in the early post-transfusion period, and anti-HBs was detected within days, persisting for the duration of the study. The mechanism of this change in HBV immune status is unknown but it may be related to the existence of HBS Ag immune complexemia detectable by radioelectro-complexing in approximately 80 per cent of HBS Agpositive blood donors. The phenomenon of apparent “cure” of the HBS antigenemic state warrants further attention.