AbstractWith use of a solenoid to generate uniform time‐varying electric fields, the effect of extremely low frequency electric fields on osteoclast‐like cell formation stimulated by 1,25(OH)2D3was studied in primary murine marrow culture. Recruitment of osteoclast‐like cells was assessed by counting multinuclear, tartrateresistant acid phosphatase positive cells on day 8 of culture. A solenoid was used to impose uniform time‐varying electric fields on cells: sham exposures were performed with an identical solenoid with a null net electric field. During the experiments, both solenoids heated interiorly to approximately 1.5°C above ambient incubator temperature. As a result of the heating, cultures in the sham solenoid formed more osteoclast‐like cells than those on the incubator shelf (132 ± 12%). For this reason, cells exposed to the sham solenoid were used for comparison with cultures exposed to the active coil. Marrow cells were plated at 1.4 × 106/cm2in square chamber dishes and exposed to 60 Hz electric fields at 9.6 μV/cm from days 1 to 8. Field exposure inhibited osteoclast‐like cell recruitment by 17 ± 3% as compared with sham exposure (p<0.0001). Several variables, including initial cell plating density, addition of prostaglandin E2to enhance osteoclast‐like cell recruitment, and field parameters, were also assessed. In this secondary series, extremely low frequency fields inhibited osteoclast‐like cell formation by 24 ± 4% (p<0.0001), with their inhibitory effect consistent throughout all variations in protocol. These experiments demonstrate that extremely low intensity, low frequency sinusoidal electric fields suppress the formation of osteoclast‐like cells in marrow culture. Thein vitroresults supportin vivofindings that demonstrate that electric fields inhibit the onset of osteopenia and the progression of osteonecrosis: this suggests that extremely low frequency fields may inhibit osteoc