Seven alloantisera were selected showing the following characteristics: after platelet absorption they were positive with PHA-activated cells and negative with resting T and B lymphocytes from the same donors; this reactivity could be blocked by pretreatment of cells with a turkey anti-human beta 2-microglobulin serum and was not completely removed by further absorption with high doses of pooled platelets or with T or B lymphocytes from the positive donors; the reactivity of some of these antisera correlated with HLA-A antigens in a panel of PHA-activated cells and segregated with HLA haplotypes in families. It is still to be proven if the HLA determinants recognized on PHA-activated cells by these antisera are new epitopes of A,B,C molecules appearing upon activation or if they are carried by products of a new polymorphic locus linked to the HLA system.