ChT1, a member of the Ig superfamily with one V-like and one C2-like domain, is a T cell antigen that is expressed on the surface of embryonic thymocytes (day 10). In young chickens, about 90% of the thymocytes as well as a subpopulation of peripheral lymphocytes (which represent recent thymic emigrants) are ChT1 positive.1-4 Expression in the periphery declines with age and, in correlation with partial thymectomy, indicates that ChT1 can be used as an accurate marker for studying thymic function.3 This antibody can block T cell differentiation in vitro, in thymic organ cultures and in thymocyte precursors cultured on stromal cell monolayers.4