WILLIAM STUART, MD: I do recommend switching treatment if there are neutralizing antibodies. My opinion is that high probability is that neutralizing antibodies are rendering the drug, if not totally, at least partially ineffective. What I switch them to depends a lot on what they were on. There is some evidence that you can go from one of the interferons to another one -- particularly if they're less capable of producing antibodies. For example, switching from Betaseron to Avonex, that the antibody levels will fall and may fall to a range that are not, not meaningful. So it's a little hard to switch the other way: To go from a less antigenic drug to a more antigenic drug. The probability there is you're going to run into the same trouble.