Methods are presented for the staining of blocks of tissue with trivalent indium so that good contrast and good specificity for nucleic acids is achieved for the electron microscope. The tissue is fixed in organic fixative, dehydrated, subjected to reduction by lithium borohydride, acetylated by acetic anhydride, stained with trivalent indium dissolved in organic solvent, and embedded. The embedding material may be either Vestopal or butyl methacrylate especially handled to eliminate the "explosion" phenomenon. Numerous new problems encountered are discussed and a brief description of the findings is included.

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