Figure 3 Primary tumour cells isolated from colorectal cancer biopsies expressed carcinoembryonic antigen (CEA) in vitro. Tumour cells were isolated from tumour specimens obtained during surgery and cultured for short time periods in vitro. Expression of CEA was monitored by means of immunohistochemical procedures using the anti‐CEA antibody 1C3, as described in material and methods. Note the natural heterogeneity of CEA expression within the tumour cell population. Monitoring of CEA on established CEA+ LS174T colorectal cancer cells and CEA− Colo201 cells is shown for comparison.