Total elimination of serum versus serum reduction will be determined by the end application of the cell-based product. For therapeutic applications, it is best to totally eliminate serum for variability and safety reasons. If you find that you are having difficulty in removing serum completely, I would be interested in talking with you further to see if we can solve your issue. Simply reducing serum in a medium is a viable alternative if the end use allows (i.e. R&D). There are a few advantages to having a reduced serum medium versus a serum free medium. Namely, it is generally well known that cells cultured in serum are more durable and can withstand environmental insults (i.e. temperature and pH shifts during handling) slightly better than when cultured in serum free media.