Electrospun ultrafine silica fibers were calcined at 150-800 A degrees C. The relation of calcination temperature to the ability to form biomimetic apatite in a simulated body fluid solution (SBF) was evaluated. The largest apatite particles, formed on non-calcined fibers after 1 week of soaking in SBF, were 10 mu m in diameter, had a narrow size distribution (coefficient of variation 9%), and were similar to pearls on string. The particles size decreased with increasing calcination temperature below 250 A degrees C and the particles formed on the fibers calcined at 250 A degrees C were 4.5 mu m in diameter. No particles were found on those calcined above 500 A degrees C. By using a concentrated SBF at 1.5-times higher ionic concentrations than SBF, the size of apatite microparticles increased about 50%. The fibrous substrate covered with apatite particles was effective for osteoblastic differentiation of pre-osteoblastic cells.