Hepatitis B surface antigen (HBsAg), the corresponding antibody (anti-HBs), the antibody to hepatitis B core antigen (anti-HBc), and the cellular immune response to purified HBsAg (leucocyte migration inhibition test: LMT-HBsAg) were determined in 19 staff members, 11 long-term haemodialysis patients, and 22 renal transplant patients in a haemodialysis-renal transplantation unit. Past or present infection, as expressed by the presence of circulating anti-HBs or HBsAg, was found in 30 cases (58 per cent) (11 staff members, six dialysis patients and 13 transplant patients). Anti-HBc was found in all the ten HBsAg positive cases--all patients--and in 15 out of 20 anti-HBs positive cases. Four staff members and seven patients had positive LMT-HBsAg; of these only one patient had HBsAg, whereas seven cases had anti-HBs. Neither the antibody determination nor the leucocyte migration inhibition assay disclosed any significant difference between patients and staff which could explain the different course of the hepatitis B virus infection in the two groups.