Morcellised bone allograft in an in vitro model was used to test the tendency towards cup rotation using small or larger fragment sizes of an impacted graft bed and different fat contents. A cup cemented on an impacted graft bed of larger sized bone chips from the Howex bone mill containing natural marrow fat was loaded eccentrically until the cup rotated. This required 3450 N. The same chips impacted in the same way, but defatted with warm saline solution required 7000 N for rotation. The smaller sized bone chips from the Tracer bone mill defatted in a similar manner required only 1950 N for cup rotation. The conclusion was that larger sized chips, partly defatted, prevent rotation of a cup cemented on a graft bed.