Down syndrome (DS) was studied in terms of immunological function and serological aspects. It was found that antibody levels against rubella and pertussis in the sera from 36 institutionalized DS patients were comparable with those of healthy controls while low antibody levels were detected against mumps and measles. Six tumor markers were also assayed in the serum from DS patients and the serum concentration of alpha-1-acid glycoprotein (alpha 1-AGP) and immunosuppressive acidic protein, an analogous glycoprotein of alpha 1-AGP, were significantly higher than those of the control group. Multivariate discriminant function was constructed based on the concentration of tumor markers. The function could discriminate between the two groups at a sensitivity of 92.3%. Flow-cytometrical analysis has revealed that helper T level of DS patients' sera was lower than that of the control group.