Four published in vitro assays which measure cell cytotoxicity were compared utilizing murine tumor necrosis factor. These included determination of residual cell number by crystal violet staining in the presence and absence of actinomycin D, lack of viability as determined by neutral red uptake, and [3H]thymidine release in cytotoxin treated L929 cells. Treatment of cells with actinomycin D followed by crystal violet staining was the most sensitive method measured. However, addition of actinomycin D to the neutral red uptake assay could be shown to be even more sensitive. Additionally, it was shown how actinomycin D dosage, cell seeding density and time of incubation affect TNF titer.