I believe another reason for washing away serum-containing media before
trypsinizing is that among the many proteins present in serum are a number
of protease inhibitors, both large (e.g., alpha-2-macroglobulin) and small
(um, no names come to mind). So there's both the problem of lots of
meaningless substrate, like BSA, plus these inhibitors. In the end, the
effect is the same -- you require more trypsin to get release of the
Lou Hom >K '93 "Into each life a little rain
lhom at nature.berkeley.edu must fall, even in San Diego."
http://www.ocf.berkeley.edu/~lhom -- from "My Blue Heaven"