The purpose of this study was to detect the failure of passive transfer (FTP) in foals of a veterinary hospital of Perú, using the glutaraldehyde coagulation test and the measurement of total serum protein concentration by refractometry, and to evalúate the concordance between both tests. Thirty-one foals with 24 hours of age were selected for this study. From each foal, 3 mi of blood was extracted by jugular venipunc- ture, from which 1 to 1.5 mi serum was obtained. For the glutaraldehyde coagulation test, 0.5 mi of blood serum and one drop of 10% glutaraldehyde were used, and for the refractometry test, 2 drops of blood serum were necessary. Of 31 foals evaluated, 8 (25.81%) and 9 (29.03%) foals were detected with FPT using the glutaraldehyde coagulation test and refractometry, respectively. Therefore, the kappa índex was 0.93. Both tests can be used to diagnose FPT because they are simple, cheap and with immediate results.