The D-type cyclins are positive regulators of the G1 phase of the mammalian cell cycle. Cyclins D1 or D2 are over-expressed in several types of cancer, transform rodent cells in culture and therefore harbor hallmarks of cellular proto-oncogenes. In contrast, no data on expression of cyclin D3 in tissues and tumours are presently available. We have raised monoclonal antibodies (MAbs) specific for cyclin D3 and examined abundance and subcellular localisation of this G1 cyclin in a series of human cultured cell types and in 180 primary tumours of diverse histogenesis. Cyclin D3 localised predominantly in nuclei of normal and tumour cells both in culture and in situ, and a pronounced cell-to-cell variation of its abundance was reminiscent of cyclins D1 and D2. Immunohistochemical analysis of tumour and corresponding normal tissues showed strong aberrant accumulation of cyclin D3 in a subset (about 10%) of breast carcinomas, whereas only weak-to-moderate expression was found in colorectal, head and neck and uterine carcinomas, melanomas and soft tissue sarcomas. The specificity of the immunohistochemical data was confirmed by immunoblotting analysis of tissue and tumour lysates. Our results indicate that over-abundance of cyclin D3 is considerably less frequent than that of cyclin D1, yet we identify subsets of breast tumours, and potentially lymphomas, as candidate tumour types with elevated cyclin D3 expression.