BACKGROUND In newborn foals the absorption of colostrum immunoglobulins in the small intestine is maximal up to 8 hours after birth and then progressively decreases to become null after 24 hours post-partum. Thus, equine practitioners need a simple, quick, inexpensive and reliable field test to identify foals affected by failure of passive transfer rather than an accurate method yielding quantitative results within the whole range of immunoglobulin concentrations. OBJECTIVE As the validity of the immunocrit method to detect failure of passive transfer in foals had not been evaluated before, the objective of this study was to test the ability of this method to detect the concentration of immunoglobulins in a large number of foal serum samples. METHODS Assay validation using samples collected for clinical purposes. METHODS The immunocrit test, using a 40% ammonium sulphate solution, was used to measure the concentration of immunoglobulins in serum samples from 211 newborn Thoroughbred foals. The results were compared, by statistical analysis, with those of agarose gel electrophoresis, a reference quantitative method. RESULTS The values obtained by the immunocrit method were significantly correlated (R = .871; P < .001) with those measured by agarose gel electrophoresis. A cut-off value of 8 g/L of serum immunoglobulins by agarose gel electrophoresis and its equivalent of 9.5% for the immunocrit test was indicative of failure of passive transfer. The sensitivity and specificity of the immunocrit method at this cut-off point were 94% (95% CI, 90-97.3) and 82% (95% CI, 72.13-91.8) respectively. CONCLUSIONS Variable times of sample extraction after colostrum suckling, over the study period. CONCLUSIONS The immunocrit test provides a quantitative, quick, inexpensive, reliable and objective method to detect failure of passive transfer of maternal immunity in newborn foals, which is easy to perform directly in horse farms, with minimum laboratory equipment.