Hollinger, F. B., R. K. Goyal, T. Hersh, H. C. Powell, R. J. Schulman and J. L. Melnick (Baylor College of Medicine, Houston, Texas 77025). Immune response to hepatitis virus type B in Down's syndrome and other mentally retarded patients. Am J Epidemiol 95: 356–362, 1972.—A high prevalence of Australia (Au) antigen (20%) and antibody (26%) was detected in the sera of 160 Down's syndrome (DS) residents at a state mental institution in Texas. To determine if the DS patients were more susceptible to infections with hepatitis B virus, 33 patients with other mental disorders were individually matched with 33 DS patients. Au or anti-Au occurred with similar frequency in each group indicating equal exposure to the agent. Examined separately, Au antigen was observed in 36% of the DS patients compared to only 12% of the other patients. Conversely, anti-Au was detected in only 18% of the DS patients in contrast to 45% of the other patients. Thus, among equally exposed residents an immunologic difference in the host response was observed.