Several monoclonal antibodies have been raised in the past which react with human plasma cells but they have all shown additional reactions with other cell types. In this paper we describe a new monoclonal antibody, ZB51, which recognizes an intracellular antigen in normal plasma cells in cryostat tissue sections and cell smears, and which also reacts with neoplastic cells in most cases of myeloma and with plasma cell lines. The antibody shows minimal reactivity with a few eosinophils and myelocytes in bone marrow and stains a myeloid cell line. Whilst normal epithelium is not labelled, antibody ZB51 stains two carcinoma cell lines. Although it was not possible to characterize the target antigen in terms of molecular weight, the reactivity of antibody ZB51 with normal and neoplastic plasma cells makes it a useful new immunocytochemical reagent.