The relationship of serum carcinoembryonic antigen (CEA) levels to tumour size and antigen content was studied in artificially immune-deprived mice bearing human colonic, breast and lung tumour xenografts. Size was measured as in vivo volume and tumour weight at post-mortem. A multiple implant technique combined with early harvest was used to minimize centrilobular tumour necrosis. CEA was extracted from resected tumours with perchloric acid. A radioimmunoassay using chemical precipitation was used to estimate CEA in blood samples. A correlation was found between CEA blood levels and tumour size in half the tumour lines, in contrast to a recent report (Lewis & Keep, 1981). The CEA content was found to be constant for one tumour line but not another. The possibility that central necrosis in xenograft tumours may account for the discrepancies is discussed. There may be serious limitations for the use of xenograft tumour models for studying the biology of CEA.

A hypothesis is proposed that tumour lysis may be an important factor affecting blood levels of CEA. This has been explored in an experimental study with a model tumour system, consisting of immune-deprived mice bearing human CEA-producing tumours. Using agents such as irradiation, chemotherapeutic drugs, diphtheria toxin and techniques such as cryosurgery, it has been shown that tumour lysis is important when it is both rapid and extensive. The extent to which this may occur in patients remains uncertain, except in rare instances of dramatic response of malignant disease to treatment.

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