What are you looking to do?
Do you mean like a "guitar to midi controller"?
http://audioen.terratec.net/modules....=article&sid=1
These take a special pickup that gets an independent signal from each string. There can be problems with the controller figuring out the pitch and converting it to a midi note.
These trigger best with guitar strings tuned an octave higher, then you can lower the pitch with midi. They also work best with a pick. Some bass players seem to get them to trigger a regular bass fine. Once it's in midi, you can trigger any sound, a sampler or synth. You can also get it to write out the music for you.
Another alternative is to "model" your sound. This uses your bass as a signal generator and the software models different instruments. It's not triggering sound, so there's no issue with conversion, but you can't take advantage of midi triggering and music conversion.