The concentration of the acid and alkaline phosphatase (Acid P, Al-P) in cyst fluid, in bone marrow and in peripheral blood (serum) were evaluated in 12 cases of simple bone cysts (SBC). The highest concentration of Acid P and Al-P was observed in cyst fluid, and the concentration in bone marrow was higher than serum, except two cases of huge SBC's where the concentration of Al-P were decreased markedly in the cyst fluid. In beagle dogs, variable difference of Acid P or Al-P concentration in any bone marrows was observed in either proximal or distal extremities. The concentration of glucose, electrolytes, transaminase, etc in cyst fluid, bone marrow and serum was almost equivalent in a SBC. The etiology of SBC is reportedly the impaired evacuation of the cyst fluid. However, the cyst fluid presumably circulates with the bone marrow or the peripheral blood. Lower concentration of the phosphatase in serum is probably due to diffusion from higher concentration in bone marrow. However, the high concentration of the phosphatase in cyst fluid is presumably the release of Acid P by osteoclasts, or Al-P by osteoblasts. Therefore, when the activity of osteoclasts around the cyst is overwhelming, SBC may enlarge, or when the activity of osteoblasts around the cyst is overwhelming, SBC may subside.
