The hypothesis of new class I antigens has been postulated in man, and several antigen systems have been proposed: HT (Gazit), TC (TCA, TCB) (Van Leeuwen). The present study describes a new class I antigenic marker system, expressed selectively on PHA-activated T lymphocytes and on lymphoblastoid B cell lines. These markers correlated at the cellular activation stage, have been called: human activation or HA markers. 7% of the sera from multiparous women present anti-HA antibodies. The definition of class I molecule (dimer 41K - 12K) has been established by a structural analysis of the molecule; the responsible gene, located on the 6th chromosome could be close to the HLA-A gene. The equivalence with the mouse Qa markers is postulated, but remains to be totally demonstrated.