The redistribution with the time of concanavalin A (Con A) receptors at one pole of the cell after addition of FITC-Con A – so-called capping – in the peripheral blood lymphocytes from 13 aged subjects (mean: 84 ± 1 years old), and of 16 noninstitutionalized patients affected by a syndrome of precocious aging, such as Down’s syndrome (mean: 17 ± 2 years old), was studied and compared with a group of 15 normal young people (mean: 23 ± 2 years old). An opposite alteration in the percentage of capped cells, i.e. a decrease in aged subjects and an increase in Down’s syndrome patients, was observed. A derangement of lymphocyte membrane fluidity appears to be present in both groups even if the underlying biochemical defect may be different. However, a similar alteration of the kinetics of the phenomenon was present either in aged or in Down’s syndrome subjects. Both groups did not show any significant increase with time of the percentage of capped cells, suggesting that they were lacking a lymphocyte subpopulation(s) which start capping later.