The activity of lactate dehydrogenase and its isoenzymes in normal horse serum was studied under four conditions of storage. For storage periods up to three weeks freezing (-20 degrees C) was most satisfactory whereas for 24 to 48 h room temperature proved optimal. The most unsuitable temperature for lengthy storage was 4 degrees C. Storage of clotted blood for up to 48 h may result in haemolysis and unexpected rise in activity.