Exposure of normal resting B lymphocytes to EBV in vitro leads to activation, subsequent immortalization, and the establishment of lymphoblastoid cell lines (LCLs). The endemic form of Burkitt's lymphoma (BL) is associated with EBV. EBV-positive BL lines maintain the original tumor phenotype (group I BL) initially and express only one EBV-encoded protein, EBV nuclear antigen (EBNA)-1. Most of them drift toward a LCL-like phenotype during in vitro culturing and express all nine EBV-encoded growth transformation-associated proteins (group III BL). Cyclin D2 and D3 have been found previously to differ in their mRNA expression in BL and LCL. Cyclin D2 expression has been attributed to EBV gene expression and to EBNA-2 and EBNA-5 in particular. We have studied cyclin D2/D3 expression in larger series of LCLs, BL lines, and freshly EBV-infected peripheral blood B lymphocytes, both at the mRNA and protein levels. The predominant cyclin D2 expression in the LCLs and group III BLs correlated with an activated B-cell phenotype. In contrast, only cyclin D3 was expressed in the group I BL lines. EBV-carrying group II BL lines that retained the BL-associated CD10 did not express cyclin D2. A collection of in vitro EBV-converted sublines of originally EBV-negative BLs that expressed a full set of the growth transformation-associated EBV proteins maintained their CD10 and cyclin D3 expression. Blood B lymphocytes expressed no D2 or D3 as judged by immunostaining. Cyclin D2 could be detected by immunostaining in a proportion of the activated blasts 40 h postinfection. Cyclin D3 that is expressed at a low level in the established LCLs was stained easily in B blasts at this time. The level of cyclin D3 at 72 h postinfection was two to three times higher than in the exponentially growing LCLs, as detected by immunoblotting. It was still 5-10 times lower than in group I BLs. Mitogen stimulation of primary B cells induced cyclin D3 at a similar level as in the LCLs. Our data are consistent with the notion of cell lineage-specific D-type cyclin expression. They also indicate that B cells may switch their D-type cyclin expression during differentiation.