NK cells from three donors with a NK (CD3- CD56+ CD16+) lymphocytosis of unknown aetiology showed differential expression of CD45R0, an isoform of CD45 not expressed by NK cells from normal donors unless stimulated to proliferate in vitro. For donor FC, 60% of NK cells expressed CD45R0 over a 16 month period during which there was a partial resolution of the NK lymphocytosis. For donor SW, 37% of NK cells expressed CD45R0, increasing to 87% over a 14 month period during which the NK lymphocytosis increased. For donor RN few if any NK cells expressed CD45R0. After in vitro proliferation, 100% of NK cells generated from all donors expressed CD45R0. For donors FC and SW, CD45R0 remained expressed on more than 90% of cells at 3-4 weeks following cessation of proliferation. By contrast CD45R0 expression was gradually lost during long-term culture of NK cells from donor RN, with 58% of NK cells regaining the pre-culture CD45R0- phenotype. NK cells from normal donors also varied in the extent to which activation acquired CD45R0 was lost during long-term culture. The results obtained are consistent with the notion that NK cells from the NK lymphocytosis donors studied have previously undergone proliferation in vivo.