A simple method of determining cellular viability, DNA content, and isotopic labeling is described. Cells are sedimented onto slides and fixed in a mixture of trypan blue and paraformaldehyde which stains nonviable cells a dark blue color, whereas viable cells exclude the dye. The staining pattern is stable for at least 6 months and persists after the Feulgen reaction, allowing the selection of viable cells for cytophotometric DNA determinations. The authors ascertained cellular incorporation of a 3H-thymidine label by photomapping cells prior to cytophotometry and relocating these cells after preparation of autoradiographs. The method gives accurate quantitative DNA and labeling values of mouse thymocytes labeled in vitro and may be of value in studies of tumors containing a large proportion of necrotic cells.
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