With 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)2D3 was studied in primary murine marrow culture. Recruitment of osteoclast-like cells was assessed by counting multinuclear, tartrate-resistant 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 degrees 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 x 10(6)/cm2 in square chamber dishes and exposed to 60 Hz electric fields at 9.6 muV/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 E2 to 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. The in vitro results support in vivo findings that demonstrate that electric fields inhibit the onset of osteopenia and the progression of osteonecrosis; this suggests that extremely low frequency fields may inhibit osteoclast recruitment in vivo.