Monoclonal antibodies raised to a number of microfilament-associated proteins were shown to recognize the appropriate proteins in extracts from human colon tissue. They were then used in an immunohistochemical study of normal colonic mucosa, adenomas, and adenocarcinomas. A strong reaction was seen in stromal cells within the tumours (both adenomas and adenocarcinomas) when frozen sections were stained with antibodies to filamin and caldesmon. In addition, a similar reaction was seen in the adenocarcinomas when stained with antibodies to talin and gelsolin. We believe that immunohistochemical staining with these antibodies reveals a tumour-induced process in the surrounding cells, possibly related to a host response to tumours.