Polycomb group (PcG) proteins are involved in the stable transmittance of the repressive state of their gene targets throughout the cell cycle. Mis-expression of PcG proteins can lead to proliferative defects and tumorigenesis. There are two separate multimeric PcG protein complexes: an EED-EZH2-containing complex and a BMI1-RING1-containing complex. In the normal human follicle mantle, both PcG complexes have mutually exclusive expression patterns. BMI1-RING1 is expressed, but EZH2-EED is not. Here, we studied the expression of both complexes in six cases of mantle cell lymphoma (MCL), which is derived from the follicle mantle. MCL cells can be cultured in vitro and stimulated to proliferation. We found that resting MCL cells expressed BMI1-RING1, but not EZH2-EED, like normal mantle cells. Proliferating MCL cells, however, showed strongly enhanced expression of EZH2. Also, BMI1 and RING1 continued to be expressed in proliferating MCL. This is the first demonstration that EZH2 expression can be upregulated in fresh lymphoma cells. To test whether the enhanced EZH2 expression was causal for the increased proliferation in MCL, we overexpressed EZH2 in two different cell lines. In the B cell-derived Ramos cell line, EZH2 overexpression caused an increase in the proliferation rate. This suggests a possible causal effect between EZH2 upregulation and increased proliferation in haematopoietic cells.