Serial hepatitis B antigen (HBAg) serum screenings were performed in 98 cadaver kidney transplant recipients: 51 were persistently found HBAg-negative, while 47 became HBAg-positive, the majority of them acquiring HBAg postoperatively. In 28 patients HB antigenemia persisted > 6 mo. postoperatively; 1 and 2 yr graft survival rates were significantly greater in HBAg-positive than in HBAg-negative recipients. HBAg tolerance may be due to the ill-defined processes which determine kidney graft tolerance. [The role of humoral and cellular immunity is uncertain.].