The definitive diagnosis of pernicious anaemia (PA) in the elderly is by no means always straightforward, particularly when inappropriate medication has been introduced before the institution of specific investigatory procedures. A detailed haematological study was carried out on 301 patients aged 60-95 with a serum B(12) concentration at the laboratory's lower level of normal of 150 ng per litre (Euglena gracilis assay). The diagnosis of PA was based on strict predetermined haematological criteria. All patients were subsequently studied by the simultaneous oral administration of the dual isotopes (57)Co-labelled B(12) bound to intrinsic factor and free (58)Co-labelled B(12) (Dicopac test), and urine was collected over 24 hours after an intramuscular dose of 1 mg nonradioactive B(12) for estimation of the (57)Co/(58)Co B(12) ratio; 255 patients satisfied all criteria for final analysis. The Radiochemical Centre, Amersham suggests an upper limit of the normal range for the (57)Co/(58)Co ratio of 1.3 with a lower limit for PA of 2.0. We were unable to show a sharp borderline in the (57)Co/(58)Co B(12) ratio between those patients shown by other criteria to have PA and those who do not have PA; 34% of the 71 established patients had a ratio below 2.0. From our series a ratio borderline drawn at 1.4 gave only one false negative (1.4% of the PA group). Of the 175 non-PA cases, nine (5%) gave false positive results; four of these had (58)Co excretion levels high enough to make misdiagnosis unlikely. In a proportion of patients the (57)Co/(58)Co B(12) ratio was estimated at regular intervals for 36-hour periods. Maximum accuracy of isotope measurement on a single specimen was obtained 8-20 hours after isotope dosing. The Dicopac investigation is a useful simple screening test in the differential diagnosis of patients with a megaloblastic bone marrow and combined low serum B(12) and folate concentrations. When carried out by the standard technique, the degree of discrimination between normal and abnormal ratios is of limited diagnostic significance in one-third of patients.