The aim of this research work was to determine the best possible, rapid test procedure to diagnose HGG in the neonatal foal and to evaluate the course of serum IgG concentration after plasma transfusion. Two different studies were conducted for this purpose. The first study involved 119 foals between one and six days of age. The study evaluated and assessed three rapid tests fort he diagnosis of hypogammaglobulinemia (HGG). The SNAP test, the total protein determination and the determination of TP-A were investigated. The results were compared with an in-house developed quantitative ELISA as a reference method. The second study investigated the course of serum IgG concentration after plasma transfusion of newborn foals. For this purpose, 23 foals with HGG were examined at an age of 24 to 72 hours and were treated with a plasma transfusion. Serum IgG concentrations were measured before transfusion and one hour as well as 24 hours after transfusion. The following relevant results were obtained in the two studies: The first study showed that the SNAP test, the determination of TP and the calculation of TP-A provided similarly good values compared to ELISA as the reference method in the determination of IgG concentration ?800 mg/dl. Based on the 95% confidence intervals, the SNAP test and the calculation of TP-A appear to perform similarly well but better than the determination of TP fort he detection of IgG concentrations <400 mg/dl. In the second study, it was found that transfusion of 1 mg IgG caused an average increase in serum IgG level of 0.03 mg/dl (0.001-0.268 mg/dl). When the serum IgG level was controlled after 24 hours, the same amount caused an increase of 0.05 mg/dl (0.002-0.537 mg/dl). Approximately 22% oft he foals showed no measurable increase in serum IgG concentration after plasma transfusion. Based on these study results it was possible to determine which ist he most suitable test for a rapid diagnosis of hypogammaglobulinemia. Furthermore, it was shown that neonatal foals react very individually to a plasma transfusion and that a follow-up measurement after a plasma transfusion is absolutely necessary.