Although other investigators have found excellent response rates to the hepatitis B vaccine, we report here an unusually low rate of seroconversion following hepatitis B vaccination in a group of apparently healthy medical center personnel. Only 67% of these individuals developed adequate postvaccination antibodies to HBsAg, in contrast to 85 to 96% in other studies. A significant decrease in seroconversion with increasing age was noted with a 54% seroconversion rate in vaccines over the age of 40, all of whom had received gluteal injections. Employees at another facility had been given deltoid injections from the same vaccine lot and had an overall seroconversion rate of 90%. Subsequently, nonresponders from the first group were revaccinated. Seven of the ten individuals tested developed anti-HBs. We believe the relatively low rate of seroconversion in individuals above the age of 40 may have been related to gluteal injection of the hepatitis B vaccine, and further investigation is warranted.