Mouse monoclonal antibodies were produced against the bacterial product encoded by human PRAD1/ cyclin Dl gene, which is known to be involved in tumors with translocation or amplification at BCL‐1 locus of 11ql3. The immunizing antigens used were GST‐PRAD1 and T7 gene 10‐PRAD1 fusion products. Four antibodies were reactive with both PRAD1 fusion products and cell lysates of B‐cell tumor cell lines with t(ll;14)(ql3;q32) and a breast cancer cell line with 11ql3 amplification, on immunoblotting. An immunofluorescence study showed that only one of them stained nuclei of cells with 11q13 abnormalities. Since this antibody proved applicable for conventional paraffin‐embedded tissue sections, immunohistologic staining of various lymphoma tissues was performed. Eight of 11 mantle cell lymphomas showed intermediate to strong positivity and 6 of the positive cases demonstrated characteristic staining patterns that were either predominantly nuclear or both nuclear and cytoplasmic. The nuclear staining pattern was not observed with other types of lymphoma and thus may correlate with PRAD1 mRNA overexpression.