The cyclin D1 gene can be transcriptionally activated in lymphoid tumours as a result of chromosomal rearrangements but is normally silent in B and T lymphocytes. By isolating cyclin D1-related cDNAs from a B-lymphoid cell line, we identified clones that contain the coding sequences of human cyclin D2. The predicted 289 amino acid protein shares 63% identity with human cyclin D1. Although cyclin D2 transcripts were detected in many lymphoid cell lines, it was not ubiquitously expressed and there was no apparent correlation with cyclin D1 levels. For example, two B-cell leukaemia lines, JVM-2 and Karpas 620, both of which have 11q13 translocations and express cyclin D1, contained markedly different amounts of cyclin D2. An obvious distinction between these cells is that the JVM-2 line, which expresses high levels of cyclin D2, was immortalized by Epstein-Barr virus (EBV). We subsequently found that cyclin D2 is consistently expressed in group III Burkitt's lymphoma (BL) and in lymphoblastoid cell lines immortalized by EBV, but not in group I BLs, in which expression of the EBV genome is more restricted. The data imply that the D-type cyclins may have non-overlapping functions at specific stages of lymphocyte differentiation and that the expression of cyclin D2 may be influenced, directly or indirectly, by EBV.