The antigenic relationships of the human group I pepsinogens (Pg 1 through Pg 5) to each other, to the group II pepsinogens (Pg 6 and Pg 7) and to the slow moving protease (SMP) were examined. Rabbit antiserum to a preparation containing Pg 2, Pg 3 and Pg 4 formed precipitin arcs of identity against Pg 1 through Pg 5, but did not react against Pg 6, Pg 7 or SMP. The specificity of the antibodies for the group I pepsinogens was confirmed by immunoabsorption. The results indicate that the group I pepsinogens, although electrophoretically heterogeneous, are antigenically similar, and that the group I pepsinogens are immunologically unrelated to the group II pepsinogens and to the SMP.