With increasing emphasis on the early detection of cancer, the search is on for reliable markers that will be clinically helpful in the diagnosis of small tumours and in the assessment of their metastatic potential. This report presents evidence that an abnormal pattern of activity of the CD44 gene is a promising candidate for both of these purposes in various types of malignancy. By a mechanism known as alternative splicing this gene can produce different messenger RNA molecules (transcripts) which are detectable, after amplification, as separate bands in electrophoretic gels. In neoplasia many abnormal variant transcripts are produced. A previous finding in animal experiments, that one such variant might be important in metastasis, prompted our study of human tumour tissue, benign and malignant, and of corresponding normal tissues. We studied tumour tissue from 34 patients with neoplastic disease (mostly breast or colon cancer) and normal or non-malignant diseased breast or colonic tissue from 11 patients and peripheral blood leucocytes from 4 healthy volunteers. CD44 gene activity was studied by amplifying messenger RNA with the polymerase chain reaction (PCR) followed by electrophoresis and blot hybridisation. In malignant tissues there was gross overproduction of each of 9 or more alternatively-spliced large molecular variants in all samples, whereas in the control samples only the standard product was routinely detected with occasional minimal quantities of one or two small variants. Furthermore, the band pattern permitted differentiation between the 23 cases with metastatic tumours of the breast or colon and the 8 with no detectable metastases. Calibration studies seeding blood with tumour cells showed that the technique can detect as few as 10 tumour cells among 10(7) leucocytes (1 ml of blood). Analysis of CD44 splice variants may prove to have applications not just to the early detection of metastatic potential in surgical biopsy specimens but also, if our findings are confirmed, in readily available bodily fluids, to the early diagnosis of cancer in screening programmes, to the assessment of remaining disease in the body and to the early detection of recurrences.