The purpose of the study was to describe a convenient, reliable and quantitative in vitro assay system to assess the cytocompatibility of a calcium sulfate bone filler on two osteogenic cell lines and primary osteoblasts. The hypothesis was that the bone void filler, OsteoSet pellets, would not impact adversely on cell proliferation kinetics or osteogenic potential of selected cells. The hypothesis was tested by standard in vitro methodology of placing OsteoSet pellets either directly in contact with osteogenic cells, or by compartmentalizing within transwell - clear microporous membrane inserts. Data analyses were accomplished with appropriate post hoc statistics (p < or = 0.05). In the presence of the OsteoSet pellets, the cell lines exhibited a decrease in cell proliferation at days 4 and 7, independent of either cell type or tissue culture medium. A decrease in the alkaline phosphatase enzyme activity occurred in the osteogenic cell lines maintained for 9 and 16 days in the presence of the OsteoSet pellets. However, with the exception of the MC3T3E-1 line, no differences were observed with respect to calcium deposition (mineralization) by day 16. Intact human osteocalcin release data for the human-derived OPC1 line and the primary osteoblasts was inconclusive as the OsteoSet pellets may interact with the osteocalcin secreted into the tissue culture medium. The present studies describe a cell culture system to assess the cytocompatibility of bone-graft substitutes with osteogenic cells by compartmentalizing material from direct cell contact (in transwells), and additionally, by evaluating direct cell/biomaterial interactions.