MUC1 (also called: epithelial membrane antigen, EMA) represents a mucin molecule strongly expressed in various epithelia and epithelial neoplasms. Its expression correlates with clinical and pathological factors as well as prognosis in some tumor types. Additionally, MUC1 was detected in normal haematopoietic cell lines and neoplasms, especially subgroups of human lymphomas including plasma cell myeloma. Therefore, the expression of MUC1 in trephine biopsies exhibiting infiltrates of plasma cell myeloma were investigated immunohistochemically. An immunoreactivity of two monoclonal antibodies (EMA and HMFG-2) was observed in about 50% of the cases. In cases exhibiting a so-called packed marrow, EMA immunoreactivity was reduced. However, MUC1 positivity did not correlate with the cytologic grade of differentiation, the fibre content of the marrow, or survival probability of the patients. However, its strong expression in a certain percentage of cases of plasma cell myeloma may be of therapeutic impact, since new therapeutic strategies include the enrichment of MUC1-specific T cells or MUC1 vaccination.