With the usual 1% periodic acid oxidation for 5 minutes, you run the risk of 
false negatives with histoplasma.  This happen to us, so if you live in an 
area where you are likely to have this organism, you should use chromic acid 
oxidation or perhaps stronger and longer periodic acid oxidation.  Jerry 
Fredenburgh and I wrote a paper on this that was published in the Journal of 
Histotechnology a couple of years ago.  I do not have the reference handy 
right now, but will put it on histonet if anyone is interested.

Freida Carson