A study of the distribution of subtypes ad and ay among sera from hepatitis B antigen-positive subjects in North West England and North Wales revealed a marked contrast between symptomless carriers among whom ad predominated and patients with acute hepatitis the majority of whom were ay. Those with hepatitis associated with drug addiction or other forms of "needle transmission" were almost all of subtype ay. On the other hand in cases of "sporadic" hepatitis without evidence of parenteral exposure subtypes ad and ay are about equally distributed. These findings are similar to those reported from other countries in Northern Europe and North America. Although geographical and social factors clearly affect the distribution of the two subtypes it is suggested that the virus of subtype ay may be more readily transmitted than subtype ad by parenteral routes involving small amounts of blood.