This study is a 2-year follow-up of an average-risk population offered screening with both Haemoccult and Hemeselect tests to determine the interval cancer rate, and thus sensitivity. The effect on compliance with Hemeselect of testing over 1 day rather than 3 days was investigated in a separate cohort. In the first study, 3948 subjects received tests; 1489 (37.7 per cent) completed both tests and 148 had a positive result, 17 (1.1 per cent) were Haemoccult positive and 145 (9.7 per cent) were Hemeselect positive. Investigation of 142 patients revealed ten with cancer (Dukes stage A, seven; B, one; C, two). All were detected by Hemeselect but only one was Haemoccult positive. After a median follow-up of 35 (range 26-43) months, seven further patients developed colorectal cancer (stage A, one; B, three; C, three) but none followed a negative Hemeselect test (100 per cent sensitivity). In the second study 2703 subjects were offered Hemeselect tests. Compliance for testing over 1 day (48.6 per cent) was significantly better than that over 3 days (43.1 per cent) (chi 2 = 8.1, 1 d.f., P < 0.01). Hemeselect is a promising screening test for the early detection of colorectal cancer.