The effects of static electromagnetic fields (SEFs) on MG-63, a human osteoblast cell-line, were investigated. We examined proliferation, proline uptake and gene expression in an SEF approximately 1/728th the intensity of those previously reported. Cells were placed within an SEF apparatus (average field intensity of 0.618mT) with appropriate controls. Proliferation was measured by 3H-thymidine incorporation and showed a 34% decrease in cells exposed to SEF (P = .0001; N = 3). Proline, a major component of collagen necessary for bone formation by osteoblasts, incorporation was reduced 37% (P = 0.006; N = 3). Reverse-transcription-polymerase chain reaction revealed that collagen I, alkaline phosphatase, parathyroid hormone-receptor, and osteocalcin mRNA's were down regulated with the low intensity SEF. Exposure to very low SEFs affects the MG-63 osteoblasts in a manner that may be detrimental to bone formation.