Fixatives fall broadly into two categories, those which precipitate proteins and those which crosslink them.  Your sample and the desired end product will help you to determine which type of fixative to use.  Frozen tissue sections and cultured cells are often fixed with precipitating fixatives such as acetone or methanol; these fixes also help to permeabilize the sample to allow passage of solutions and antibodies into the cells.  Crosslinking fixatives, such as formaldehyde, will often produce better histology, but may require antigen retrieval or other techniques to allow antibodies to recognize epitopes or otherwise restore "native" conditions to the sample.

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