A hospital-based program to immunize adults against tetanus unless specific contraindications to immunization are present has been in effect at Parkland Memorial Hospital since 1959. Adsorbed tetanus toxoid was used from 1959 to 1970, and was replaced at that time by adult type adsorbed combined tetanus/diphtheria toxoid. In the present survey, the titers of diphtheria and tetanus antitoxins were determined in sera from 97 adults admitted to the Medical Service. Titers of diphtheria antitoxin less than 0.0125 units per ml were found in only seven patients and less than 0.0125 units per ml of tetanus antitoxin in only 17 patients. This high prevalence of immunity especially to tetanus appears to reflect our practice of routine immunization of adults. Although the recommended frequency of tetanus boosters has recently been reduced for adults who have completed a full primary immunization series, susceptibility to diphtheria or tetanus or both is not uncommon among adults in the US. Unless contraindications are present, we therefore urge routine immunization of all adults seeking medical care and of all hospital personnel as a means of diminishing the risk of diphtheria and tetanus in those adult populations with a significant incidence of susceptibility to these diseases.