High critical current density, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 2500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2<sup>, and small filament diameter, d ∽ 3 μm, are required in multifilamentary NbTi wire used for superconducting accelerator magnets. Wires obtained from various commercial sources had J<sub>c</sub>'s in the range 1000 to 2800 A/mm<sup>2</sup> and d's in the range 1 to 23 μm. The filaments were examined by means of scanning electron microscopy in order to determine the reason for the variation in J<sub>c</sub>. It was found that the filaments in high J<sub>c</sub> wires had clean smooth surfaces and uniform cross section along-their lengths. Filaments in low J<sub>c</sub> wires show formation of Cu<sub>4<sub>Ti compound particles on their surfaces and large variations in cross section. The lower critical current measured in these wires is believed to be largely due to this effect. The superconducting-normal state transition is relatively wide in these wires.</sub></sub></sup></sup>