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An improved method for identifying murine mesenchymal cells in chimeric tissues or heterotransplants using Hoechst dye 33258 is described. Following fixation in formalin-saline, tissues are embedded in JB-4 plastic. Sections 3 micron thick are then stained in a 10 microgram/ml solution of Hoechst 33258 in Hanks' balanced salt solution for 5-10 min at 4 C. After rising, the sections are coverslipped using a modified polyvinyl alcohol mounting medium. This approach offers several advantages over existing techniques: 1) uniform section thickness is more easily obtained than with paraffin or cryostat microtomy, thereby allowing improved resolution and more reliable identification of mesenchymal cells with small nuclei such as skeletal muscle myocytes or fibroblasts, 2) the preparations are stable over long periods and can be repeatedly viewed or photographed, and 3) calcified tissues can be examined without prior decalcification. An example is shown of species identification using rat chondrosarcoma cells grown in nude mice.