LDH is an intracellular enzyme, which when cells degenerate is released to the extracellular spaces and body fluids. Cells and organs in the mammalian body differ from each other with respect to their LDH isoenzyme patterns. These circumstances have led to the use of LDH isoenzyme determinations in laboratory diagnostic work. In the present investigation total LDH activity and LDH isoenzyme distribution in equine synovial fluid from healthy joints, joints with serous arthritis, osteochondrosis dissecans and arthrosis, were determined. The fluids from the diseased joints differed from normal synovial fluid with respect to total LDH activity, and the different joint diseases each seemed to give rise to a characteristic isoenzyme pattern. In order to examine possible sources of the increased LDH activity and altered isoenzyme patterns, blood plasma, red and white blood cells, synovial membrane and articular cartilage were also studied. It was found that LDH4 and LDH5 were present in high amounts in articular cartilage, and an increase in these isoenzymes was the most characteristic feature in synovial fluid from joints with arthrosis. The results were discussed in view of possible diagnostic value of isoenzyme determinations on synovial fluid.