SEF, the average increase in pit volume after 28 days culture with osteoclasts was 0.28 um um 2. This suggests a voRR of 001 um' um 2 day 1. As we have shown that for our cell culture protocol human osteoclasts only bepan resorption in the third week (101 and taking the shorter time of actual resorption into ac- count, the voRR might be as high as 0,02 jams jam day 1. Rubin et al. have reported bone formation rate of 0.08 um -2 day a pm for normal sub ects 134 As normal adult subjects do not normally gain but rather loose bone with ageing this bone formation must be balanced by a similar rate of bone resorption. Thus the derived voRR results for Ca2 obtained with human osteoclasts under max- imal stimulation with RANKL are still at least 75x lower than the observed resorption of bone in vivo, This is in line with reports on "resorbable" biomaterials, which can be found incompletely re- sorbed even years after implantation 189) These results, therefore, raise the hope that the method described may allow a more accu- rate prediction of bioresorption of biomaterials. This hope, how. ever, will first have to be validated with parallel in vitrolin vivo experiments.