An antiserum with specificity for human T lymphocytes was evaluated as a diagnostic reagent in the clinical immunology laboratory. The antiserum was used in indirect immunofluorescence to detect T cells in blood samples from normal controls and patients with various disorders involving the immune system. The same samples were also examined using two established tests for T cells, the rosette reaction with sheep red blood cells (E rosette assay) and the proliferative response to phytohaemagglutinin (PHA). The antiserum and the rosette assay detected approximately the same numbers of T cells in controls, but the antiserum indicated T cell deficiency in several patients who appeared normal by resetting or PHA reactivity. PHA stimulation in autologous plasma was depressed in a further group of patients who had normal T cell numbers by the other two tests. In vitro experiments with normal lymphocytes indicated that the E-rosette receptor was distinct from determinants detected by the T-cell specific antiserum. The immunofluorescence test with the anti-T serum provides an additional assay which appears to be more sensitive than the other two tests in detecting certain cases of T lymphocyte deficiency.