In the period January-September 1974, 50 cases of hepatitis B infection occurred among a nephrology center's hemodialysis patients and staff. The in-center patient population had an attack rate of 96%. Epidemiologic analysis of risk factors for patients revealed an association between the receipt of intravenous medication and the subsequent development of hepatitis, suggesting that parenteral inoculation was a mode of spread among patients (p equals .008). Nineteen per cent of the staff contracted hepatitis, and all of these personnel had had close contact with patients (p equals .005). The prevalence of hepatitis B infection in staff was related to the failure to use gloves (p less than .01), and accidental needle puncture was associated with the development of clinical hepatitis. These data suggested that disease was transmitted to staff by contact with contaminated blood or close personal contact with patients. Additional data showed that the presence of endogenous antibody protected both patients and staff from antigenemia (p equals .002). These data support the hypothesis that contact with blood is the primary mechanism of spread of hepatitis B in dialysis units, and suggest that, as preventive measures, gloves should be used and antibody-positive staff should dialyze antigen-positive patients.