Aberrant expression of c-myc has been implicated in the development of colorectal carcinomas. We have used monoclonal antibodies 6E10 and 9E10, raised against mid-sequence and C-terminal peptides of the c-myc protein, to study the distribution of myc protein in normal and diseased bowel at the light microscope and ultrastructural levels. Normal mucosa showed staining only of some nuclei in the proliferative zones of crypts. In adenomas, staining varied from predominantly nuclear to pancellular to focal or pancytoplasmic. Moderately well differentiated areas of carcinomas gave strong focal cytoplasmic staining, while in poorly differentiated tumours staining was pancytoplasmic. Electron microscopy with these antibodies detected myc protein associated with dense chromatin and, where cytoplasmic staining occurred, with polyribosomes. Tumours showed a reduced staining of nuclear pores compared with normal tissue. Comparison of staining patterns with 6E10 and 9E10 in normal tissue, adenomas, and tumours suggests that tumour progression is associated with an accumulation of cytoplasmic c-myc protein, perhaps resulting from alterations to the C-terminus which reduce the efficiency of nuclear targeting of the protein and thus disrupt the regulation of the cell cycle.