Comparison bewteen Fas-induced cell death detected as percentage of specific cell survival by the trypan blue exclusion test and as percentage of specific cell death detected by FACS analysis of shrunken/hypergranular cells. Both methods detected defective Fas activity in patients with ALD (n = 6) and not in those with typical chronic ITP (n = 6) (P < .01 using the nonparametric Mann-Whitney test). T cells were incubated for 18 hours in the presence of anti-Fas MoAb (A). Total cell survival was then assessed by the trypan blue exclusion test, and the proportion of apoptotic cells was detected by FACS analysis of shrunken/hypergranular cells using the FSC/SSC FACS parameters. Results are expressed as means ± SD. A significant inverse correlation was found between the two methods (r = −.84, P < .001). (B) FACS analysis of cultured T cells derived from a patient with ALD and one with ITP and treated with control medium or the anti-Fas MoAb for 18 hours. In each plot, the same gate (R1) was set between apoptotic and live cells, and the proportion of these cells is shown in the upper left and lower right corners, respectively.