A modification of the MTT based tetrazolium colorimetric assay is desc
ribed. Using the electron-coupling agent menadione formazan formation
by murine splenocytes and P-815 cells was significantly increased wher
eas dye reduction by macrophages was hardly influenced. These observat
ions suggest that it should be possible to improve the tetrazolium bas
ed cytotoxicity assays of murine macrophages against cells of the syng
eneic tumour cell line P-815.