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By limiting dilution of WEHI 164 mouse fibrosarcoma cells we have isolated a cell line, WEHI 164 clone 13, which is extremely sensitive to cytotoxic factor (CF) derived from human monocytes. By using WEHI 164 clone 13 in a MTT tetrazolium cytotoxicity assay it was found that CF supernatants from activated monocytes had to be diluted 10⁵–10⁶ times to reach the dose which produced 50% dead cells (LD₅₀). By comparing the LD₅₀ of different target cells, WEHI 164 clone 13 cells were found to be approximately 10³ times more sensitive for CF-induced cytotoxicity as compared to WEHI 164 parental cells and approximately 10² times more sensitive as compared to actinomycin D-treated L929 cells. Treatment of the WEHI 164 clone 13 cells with actinomycin D did not increase their sensitivity for CF-induced cytotoxicity. Recombinant tumor necrosis factor (rTNF) also mediated high cytotoxicity towards WEHI 164 …