Surface phenotype and functional in vitro activities were studied in 2 cases of lymphoproliferative disorders of granular lymphocytes. Cells from both patients presented the same, previously unreported, surface phenotype (i.e. T3+, T8+, T4-, HNK-1-, NK-15+, M1-), were unable to display either Natural Killer (NK) activity or suppressor function in a poke-weed-driven system, and showed a defective response to phytohemagglutinin (PHA). On the basis of available schemes for NK cells ontogenesis, we will discuss the phenotype and functional activities of patients' cells suggesting that the cell population expressing the T3+, T8+, HNK-1-, NK-15+, M1- phenotype might represent a discrete stage along the NK-cell differentiation pathway.