B-cell chronic lymphocytic leukemia (B-CLL) is considered an accumulative disease of antigen-naive CD5(+) B lymphocytes that circulate in the resting state. However, to evaluate the possibility that B-CLL cells resemble antigen-experienced and activated B cells, we analyzed the expression of markers of cellular activation and differentiation on CD5(+)CD19(+) cells from B-CLL patients and from age-matched healthy donors. The leukemic cells from all B-CLL patients, including those that lack significant numbers of V gene mutations, bear the phenotype of activated B cells based on the overexpression of the activation markers CD23, CD25, CD69, and CD71 and the underexpression of CD22, Fcgamma receptor IIb, CD79b, and immunoglobulin D that are down-regulated by cell triggering and activation. Furthermore, these leukemic cells resemble antigen-experienced lymphocytes in the underexpression of molecules that are down-regulated by cell triggering and in the uniform expression of CD27, an identifier of memory B cells. A comparison of the phenotypes of B-CLL patients with and without immunoglobulin V gene mutations suggests that the 2 subgroups differ both in specific marker expression (CD69, CD71, CD62 L, CD40, CD39, and HLA-DR) and in the time since antigenic stimulation, based on the reciprocal relationship of CD69 and CD71 expression. These findings imply that the leukemic cells from all B-CLL cases (irrespective of V gene mutations) exhibit features of activated and of antigen-experienced B lymphocytes and that the B-CLL cells that differ in immunoglobulin V genotype may have different antigen-encounter histories.