To measure the immunoglobulin G (IgG) concentration in colostrum, milk and serum samples, a sandwich enzyme-linked immunosorbent assay (ELISA) detection system was developed. The system provided high reproducibility and sensitivity for routine diagnostic purposes. The period of fluctuating serum concentrations of IgG was monitored in new-born foals and their mares for a period of 6 weeks postnatum and postpartum, respectively. All foals received colostrum from their mares. The mean IgG concentration in the precolostral mare serum was approximately 19.0 mg/ml and decreased significantly to 13.8 mg/ml within the first 24 h postpartum. The IgG value fell to a minimum of 11.2 mg/ml by day 21 and increased to 21.6 mg/ml by day 42 postpartum. Within the first 4 h postpartum, mean IgG concentrations of 54.5 mg/ml were measured in the colostrum. A significant decrease to 10.1 mg IgG/ml colostrum was then noted 9–12 h postpartum. The mean IgG concentrations in foal serum increased from 0.3 mg/ml (precolostral value) to 9.6 mg/ml within 5–8 h postnatum. After 13–16 h postnatum, the highest IgG value of 15.7 mg/ml was reached. Over time the mean IgG concentration decreased significantly to 7.9 mg/ml at day 35. At the end of the observation period (day 42 postnatum) the mean IgG concentration once again increased to 11.2 mg/ml serum. In addition, the possible influence of various parameters on IgG concentration were examined. No significant influences could be shown by the breed, mare age, number of pregnancies, days of gestation, month foaled, foal sex, or the different farms. Finally, the cumulative incidence of failure of passive transfer (FPT) defined as IgG levels < 4 mg/ml foal serum, and partial FPT (PFPT) at levels ranging from 4 to 8 mg/ml foal serum was determined. From a total of 70 foals, 10.0% showed FPT and 18.6% showed PFPT.