Analysis of peripheral blood lymphocytes by two- and three-colour high-sensitivity staining with UCHL1 (CD45RO) and other markers shows that the expression of CD45RO on lymphocyte subsets is more complex than is generally supposed. In addition to the populations which express CD45RO and RA in a mutually exclusive manner, up to 30% of cells in adult blood express both markers, at low levels. This "intermediate" population includes CD4-positive cells, and a proportion of these cells express the p55 chain of the IL-2 receptor (CD25), suggesting that they are activated. In cord blood there are few RO-bright cells, but CD45RO is expressed at low intensity on a proportion of cells. Among the CD45RO-bright cells in adult blood at least two subsets can be detected by using MHC Class II and the homing receptor L-selectin as additional markers. This complexity suggests that memory cells are a subset of CD45RO-expressing cells, but that this marker is also found on cells that are activated but not irreversibly "switched" to memory cells.