We report on the fragile site expression in lymphocytes from 16 Down syndrome (DS) men aged 24 to 52 years. Among rare fragile sites, 16q22 has been reported to be induced or enhanced by alpha-interferon. Since DS cells have three copies of the receptor for alpha-interferon, we hypothesized a possible enhancement of 16q22 expression. This fragile site has also been related to a specific rearrangement in M4 acute nonlymphocytic leukemia. In view of the high incidence of acute leukemia in DS subjects, we studied the expression of 16q22 in lymphocyte cultures treated with 5-bromodeoxyuridine or alpha-interferon. We also studied whether the repair deficiency of DS cells could affect the expression of aphidicolin-induced fragile sites. The level of chromosomal aberrations was compared with that found in aphidicolin-treated cultures from 12 normal subjects of the same age. We found neither spontaneous or BrdU-induced fragility at 16q22 nor induction by alpha-interferon. Chromosomal breakage rate was increased in alpha-interferon-treated cultures in comparison with control cultures of the same subjects. Aphidicolin-induced fragile sites expression in DS patients did not differ significantly from that found in the lymphocyte cultures from control subjects.