Due to the time-limited intestinal uptake of colostral immunoglobulins, the suggested treatment of hypogammaglobulinemia in new-born foals is usually plasma transfusion. The aims of this study were twofold: firstly, to investigate the course of serum IgG concentration after plasma transfusion in newborn foals; and secondly, to determine the amount of transfusion required for a significant increase in serum IgG concentration. For this purpose, the IgG concentration was measured in 23 foals at three different points in time: before transfusion, 1 hour after transfusion, and 24 hours after transfusion. There was an increase in IgG concentration in the blood of 18 foals (78.3%). In five foals (21.7%), no increase in serum IgG concentration were detected after plasma transfusion. Transfusion of 1 mg IgG caused an average increase in IgG level of 0.03 mg/dl (0.001-0.268 mg/dl) 1 hour after transfusion. After 24 hours, the same amount of IgG caused a larger increase of 0.05 mg/dl (0.002-0.537 mg/dl). None of the foals demonstrated adverse reactions to the plasma transfusion. These values provide a guidance how much IgG is needed to increase serum IgG concentration to a desired level. However, this study has shown that there is a high variability in serum IgG concentration after plasma transfusion. Which highlights the necessity for monitoring IgG concentration following transfusion.