Epstein-Barr virus (EBV) is causally linked with endemic Burkitt's lymphoma (BL), a tumour whose homogeneous cell surface phenotype suggests derivation from a particular subset of activated germinal centre B cells in vivo. Endemic BL also shows an unusual form of EBV in fection with downregulation of certain of the virus latent proteins which are constitutively expressed when EBV infects and transforms normal resting B cells in vitro. Here we question whether this virus:cell interaction is unique to malignant BL cells or whether it might be reproduced by in vitro infection of those particular germinal centre cells displaying the BL-like phenotype. Firstly, we show by biochemical meanns that a subset of normal tonsillar B cells does indeed express the globotriaosylceramide glycolipid BLA and the common acute lymphoblastic leukaemia antigen CALLA, 2 important markers of the BL phenotype. Secondly, using 2-colour immunofluorescence labelling with anti-BLA and anti-CALLA monoclonal antibodies (MAbs), 4 subsets of low buoyant density tonsillar B cells (BLA+ CALLA+, BLA+ CALLA−, BLA− CALLA+, BLA− CALLA−) have been separated by means of a FACS and tested for their susceptibility to EBV-induced growth transfromation in a limiting dilution assay. The BLA+ CALLA+ (i. e., BL-like) subset contained the highest proportion of cells already actively in cycle in vivo and gave the lowest yield of transformants, perhaps reflecting the greater efficiency with which EBV transforms resting target cells. Of the cell lines established from the BLA+ CALLA+ population, a significant number retained BLA expression but CALLA was always lost. In 2 further respects, these lines resembled conventional in vitro transformants rather than lines of BL type; thus the cells expressed cellular “activation” antigens (CD23, CD39, CD30, Ki-24) characteristic of the lymphoblastoid phenotype and contained the full spectrum of EBV latent proteins.