Most diagnostically valuable monoclonal antibodies recognize antigens that do not survive conventional tissue processing. The use of frozen tissue sections for immunohistologic studies overcomes this obstacle but introduces a number of practical problems, e.g., the necessity to store material in the frozen state, poor morphologic preservation, etc. In the present paper we report that antigenic denaturation during conventional tissue processing appears to occur during exposure to aldehyde-containing fixatives and to alcohol but not as a result of heating or exposure to melted paraffin wax. In consequence, we have developed a technique by which tissue is freeze-dried and then embedded directly in paraffin wax. All but one of the 40 monoclonal antibodies investigated stained the freeze-dried paraffin sections with an intensity equal to or greater than that observed on frozen sections. There was less diffusion artifact and less background staining than in cryostat sections, and cellular morphology was better preserved. One of the most important advantages of this new method is that antigens in freeze-dried paraffin-embedded tissue are stable, and tissue blocks may be handled in the same manner as conventional paraffin blocks. An additional finding was that, once the tissue has been freeze-dried, paraffin embedded, and sectioned, the antigens it contains are resistant to fixatives (e.g., formol, formol sublimate, alcohols) which would very rapidly cause their destruction in frozen sections.