The archives need to be revisited on this discussion.  Saponin has a
different kind of action on cell membranes, actually works on chloresterol
component to punch holes in membrane, and is reversible.  Tween 20 and
Triton X work differently to reduce ionic interactions and also permit
permeability of cell.  I can't explain it as well as some others.  

I never lose frozens with Tween or Triton  but I also do not use a high
concentration, no more than 0.1%, usually 0.05% for Tween 20.  This is with
acetone or acetone alcohol fixation.  Loss of sections can be reduced by
handling of sections before and after fixation, how dried, before exposing
them to detergents and high concentration of hydrogen peroxide for endog
blocking is also a big culprit. Never used Tween until I cleaned up a
multitude of sins (background staining!) with it, now it is routine. 

As for saponin, I never use this on acetone or A/A fixed sections, will
chew the begollys out of FS, I think it is really hard cells I also have to
be gentle with buffer containing saponin, using tender care when handling
the sections, no hard flow. The only way I know to keep acetone fixed
frozens exposed to saponin on slides, is to use the tape transfer method

As for the cytokine story, this is coming to light very soon, with the tip
of the iceberg just around the corner and reserve the use of saponin for
cytokine work exclusively.  
Gayle Callis
Veterinary Molecular Biology
Montana State University
Bozeman MT 59717-3610
406 994-4705
406 994-4303