Abstract Immunocytochemistry was used in parallel with conventional cytology to detect circulating malignant epithelial cells in 42 patients undergoing resection for colorectal cancer. Preoperative peripheral and peroperative mesenteric venous blood samples were taken. Tumour cells were isolated on a density gradient and cytospins prepared. Slides were stained by conventional cytology (May-Grünwald-Giemsa) and by an indirect immunoperoxidase technique with the anticytokeratin antibody KG 8.13. Using conventional cytology, definite morphological evidence of malignancy was observed in three patients and suspicious features in a further seven. Immunocytochemistry confirmed these findings in all three of the malignant but in only one of the suspicious cases. Counts of immunostained cytospins showed the concentration of tumour cells in blood samples from these four patients to be in the range 0-954 cells/ml. This study supports the use of immunological markers to detect and enumerate malignant cells. This method provides a powerful tool to investigate one aspect of the metastatic process.