Ultrathin frozen sections of fresh liver tissue were floated on actinomycin D-3H. Quantitative high resolution radioautography was performed to determine the value of the method for detection of DNA by electron microscopy. A complete series of control experiments involving various treatments of frozen sections with enzymes (pronase, DNase) and 0.1 N HCl were also carried out to determine the specificity of the labeling. The results indicate the value of the method for detection of DNA directly on ultrathin frozen sections. Short treatments with pronase followed by DNase reduce the labeling to zero, whereas removal of chromosomal proteins with HCl increases the amount of radioactivity in the nucleus considerably. The results are discussed in view of the future applications opened by ultracryotomy, since radioautographic detection of various macromolecules and cellular components by labeled compound with specific affinities will now be possible.

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