In two renal dialysis units which had outbreaks of hepatitis B, patients were studied for the presence of e antigen and antibody. In one unit, in which the illness was mild and most of the patients (78%) became carriers of HBsAg, all but one patient had e antigen. Most of the HBsAg carriers had persisting e antigen and most of the transient patients had also transient e antigen. In the other unit, in which the illness was more severe, none of the patients became a carrier. Only 30% had e antigen during the acute phase of illness and 40% developed anti-e just before clearing HBsAg. It seems possible that the difference between these two units may reflect strain differences in the infectivity of hepatitis B virus.