Guinea pig immunoglobulins have been separated into the γ1 and γ2 classes by chromatography on DEAE cellulose. Both types show charge heterogeneity. The heterogeneity of the γ2 immunoglobulins is attributed to the F(ab) fragments. As noted previously by others, the ability to mediate passive cutaneous anaphylaxis (PCA) in the guinea pig skin is confined almost entirely to the γ1 immunoglobulins. Both types of immunoglobulins participate in C-dependent allergic reactions of the immediate type. These include the generation of anaphylatoxin, the mediation of reverse passive Arthus reactions, as well as allergic histamine release from normal rabbit platelets.