A simple method was devised for applying mechanical stretching to bone cell cultures. Bone cells cultured on the flexible plastic membrane of a Petriperm dish are placed over a template with a convex surface. A lead weight is then placed on top of the dish which causes the membrane and the tightly attached cells to be stretched. Mechanical stretching, applied either intermittently or continuously for a 2-hour period resulted in a 64% increase in the number of cells synthesizing DNA. Stretching the cells also significantly increased incorporation of tritiated proline and tritiated leucine. To assay the ratio of collagenous to noncollagenous protein, medium and cell layers of cultures labeled with tritiated leucine were incubated with collagenase and the digests chromatographed on PD 10 columns. The amount of collagen synthesized by stretched and unstretched cultures did not differ; but an increased synthesis of noncollagenous proteins was observed in the stretched cultures.