HL-A antigen frequencies were examined in 76 Down's syndrome individuals and 733 normal Caucasians. 10 antigens of the first locus and 15 antigens of the second locus were defined, using a microlymphocytotoxicity technique. No significant differences were observed between the normal and Down's syndrome samples, in contrast to a previous report (Boxer and Yokoyama, 1972) of decreased HL-A antigen frequencies in Down's syndrome individuals. Our results therefore suggest that there is no relationship between trisomy 21-associated immune aberrations and altered HL-A antigen frequencies.