The diagnosis of megaloblastic anaemias caused by cobalamine or folate deficiency are still difficult. The dosage of these two substances help to differenciate between both carencies, but it is not determinant of any of them and is an expensive method. Homocisteinuria (HC), methylmalonuria (MMA) and formiminoglutamic acid (FIGLU) are cheap tests which could help in the differential diagnosis, if they are used properly. We report 62 patients to whom we made these test simultaneously. All of the patients received 10 micrograms of vit B12 and after 72 hours, 1 mg/day of folic acid (for 3 days). In both cases waiting for the increase of reticulocytyes up to 150 x 10(9)/L as a form of therapeutic test of diagnosis. By this simple way we have detected 97.9% of specificity for cobalamin deficiency of the MMA test, and only 4.2% for HC. This last test had increased its specificity up to 91.6% in association with the negative FIGLU test. We have also found a high specificity (92.3%) for FIGLU due to the detection of folate deficiency, in opposition with other authors who had described it as low as 50%. We have also compared the costs of the 3 tests with the dosage of cobalamine and folate, and we have found that the formers are 11 times less expensive than the last ones.