We present herein an improved method for the use of osmium ammine in a Feulgen-type reaction for specific staining of DNA at EM level and an analysis of its Schiff-type reagent behaviour. The activation of osmium ammine to a Schiff-type reagent (so far routinely performed by bubbling with gaseous SO2) can be accomplished by adding S0(2)-generating chemicals as for light microscopy. When used after HCI hydrolysis on epon or acrylate sections, activated osmium ammine behaves like a Schiff-type reagent, and the DNA staining can be selectively and completely abolished by aldehyde blocking agents. This preparation has the advantage of eliminating the use of gaseous SO2 thus rendering the technique more widely available to laboratories which cannot handle gas cylinders containing SO2. We recommend the use of osmium ammine in 8N acetic acid and 40mM sodium metabisulfite for 1 h at 37 degrees C for epon sections, and in 0.2N HCl and 0.2M metabisulfite for 30 min at room temperature for acrylate sections.