Monoclonal antibodies production older mice - (Jun/14/2007 )

We were working on a MAbs production project last Fall (october 2006). Balb/c mice were bought at that time and immunized using a standard protocol 0 and 21 days + a booster prior fusion for the mice used. There were some left over mice from this project that are now 1 year old. We are thinking of using them for another fusion very soon.

1) What kind of booster should I give them? 1 injection followed by the booster prior fusion 3 weeks later (or 1 week) or shall the booster prior fusion be sufficient?

2) Do you have any experience with fusion using such old animals? What were your results?

Thanks for any advice/comment