MTT colorimetric assay detects mitogen responses of spleen but not blood lymphocytes.

In an effort to simplify methods for assessing mitogen stimulation of blood lymphocytes, the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) colorimetric assay was compared with the conventional tritiated thymidine deoxyriboside (3H-TdR) incorporation assay. Although able to detect mitogen responses by mouse and rat spleen cells, the MTT assay did not reliably measure blood mononuclear cell responses in mouse, rat or man. The 3H-TdR incorporation assay was effective in every case. Modifications of the MTT assay procedure did not rectify this disparity, which was caused in part by high MTT optical density readings by unstimulated cells. The inability of the MTT assay to detect mitogen responses by blood cells seems to reflect a species-independent physiologic property of these cells.