Carrageenan, in its three fractionated forms ι, κ, and λ, is able to stimulate in BALB/c mice the production of IgE, IgG1 and those antibodies producing passive hemagglutination to ovalbumin. The adjuvant effect of carrageenan is more evident in the secondary response, and the ι form is more effective than the κ and the λ forms. The data are compatible with the possibility that adjuvanticity of carrageenan can be, at least partly, correlated to its antimacrophage activity.