NucleophosminlB23 is a 38 kD molecular phosphoprotein involved in ribosome assembly and transport. In view of the fact that nucleophosminlB23 appears to be more abundant in tumour cells than in normal cells, the mRNA expression and immunohistochemical localization of nucleophosminlB23 were investigated in 19 samples of non-neoplastic mucosa, six adenomas, and 16 adenocarcinomas of the colorectum. Northern blot analysis revealed that nucleophosminlB23 mRNA is expressed a t a higher level in adenomas and carcinomas than in non-neoplastic mucosa of the colorectum. Immunohistochemical staining of formalin-fixed, paraffin-embedded tissue sections after microwave antigen retrieval, using a nucleophosmin/B23-specific monoclonal antibody, showed almost exclusively diffuse nuclear reactivity of a majority of the epithelial cells in non-neoplastic mucosa: in adenomas, reactivity was almost exclusively nucleolar and in carcinomas, nuclear as well as nucleolar staining was observed. During mitosis, the immunoreactivity of nucleophosminlB23 appears in the cytoplasm. The results indicate that the expression of nucleophosminlB23 is higher in neoplastic than in non-neoplastic colorectal mucosa. Furthermore, the pattern of nucleophosminfB23 expression shifts from nuclear to nucleolar early in the adenoma-carcinoma sequence. The exact function of nucleophosminfB23 in colorectal carcinogenesis remains to be determined.