A radiographic study of 45 knees immediately and at least two years after operation showed that the average depth of cement penetration was 1.5-3.0 mm at different locations around the interface and that there was a strong inverse relation between the development of radiolucency and initial cement penetration. In laboratory studies of penetration in the upper tibia, penetration was approximately proportional to bone pore diameter and to the square root of the applied pressure and inversely proportional to the time after initial mixing. There was a low correlation of the tensile strength of the cement-bone bond with the depth of penetration, probably due to the variability of the trabecular bone strength. Failure occurred by direct cement pullout or by partial or total fracture through the bone. In light of all factors, the suggested ideal depth of cement penetration is 3-4 mm. This penetration can be achieved by using the left-lift method about four minutes after initial cement mixing, but with some selective prepenetration around the periphery of the tibia.