The effect of bone cell stimulatory factors can be measured with thymidine incorporation only under specific conditions.

This study examines the efficacy of using radiolabeled thymidine as a measure of bone cell DNA synthesis. The effect of a bone-active peptide on cell proliferation is assessed under different labeling conditions and it is shown that the apparent stimulation in DNA synthesis is due to an increase in participating cells and not to labeling artifacts. In another series of experiments we demonstrate how the use of carrier-free thymidine can cause erroneous results. From these data it is shown that only 10-28% of the cells in culture are participating in DNA synthesis. Therefore, under specific conditions, a 100% stimulation in thymidine incorporation by a mitogenic factor can be caused by as little as a 10% increase in the number of DNA synthesizing cells.