When cytotoxic titrations were made of rabbit anti-mouse brain and anti-Thy-1.2 sera against mouse lymphoid cells in the presence of sodium barbital or xylocaine, it was observed that a proportion of T cells were much more sensitive (50 x) than the remainder. This effect could be detected with ethidium or fluorescein diacetate as indicators of cell death, but not with trypan blue or 51Cr. The highly sensitive cells were found in all lymphoid tissues, being most numerous in spleen and least in lymph nodes.