THE hybridisation of established cell lines with differentiated cells provides a useful strategy for the production of cell lines having differentiated properties and unlimited growth potential. Clones of these hybrids could be particularly useful in the dissection of cellular interactions of the immune system, a network involving numerous specific cell types which display characteristic ensembles of surface and internal markers. We describe, here, the production and partial characterisation of hybrid cell lines with surface antigens characteristic of T lymphocytes.