Ceacam1 can be found on activated T cells [7, 8, 29 ] and, we thus performed a kinetic analysis of Ceacam1 expression on T cells during alloactivation. We adoptively transferred CFSE-labeled B6 T cells into irradiated BALB/c recipients, and observed transient expression only on day 2 after alloactivation ( Figure 6 and data not shown). Furthermore, only CFSE lo alloactivated T cells, which have divided $4 times in 48 hours, expressed low but consistently detectable levels of Ceacam1 ( Figure 6A ). These kinetics are consistent with a role for Ceacam1 in regulating early events in T cell alloactivation.