The aim of the study was to determine the suitability of basic haematological, biochemical, and gasometric tests in checking the effectiveness of transfusion therapy in foals during isoerythrolysis. The number of red blood cells, haemoglobin, haematocrit, and partial pressure of carbon dioxide, oxygen, and blood pH was determined immediately before and several times after blood transfusion. The concentration of serum free bilirubin was also measured to confirm haemolysis. Fluids (0.9% NaCl, multielectrolytic fluid, 5% glucose) and antibiotics (penicillin, amikacin) were provided to the foals. The lowest values of haematological parameters were observed before transfusion. This was accompanied by decreased partial pressure of oxygen, low pH, and increased arterial carbon dioxide tension. Transfusion of whole blood led to a gradual normalisation of the haematological parameters, also accompanied by the normalisation of gasometric indicators (decrease in pCO2 and pO2 and pH increase). Monitoring of selected haematological and gasometric parameters allows to evaluate the efficacy of blood transfusion during treatment of haemolytic disease of foals.