Recent studies suggest that the I-region may control two separate classes of antigens, one class consisting of protein-defined antigenic determinants and the other consisting of carbohydrate-defined specificities. It was important to determine the contribution of these two classes of Ia antigens to various I-region controlled immune phenomena. Initially, the nature of the Ia antigens carried by antigen-specific helper factors secreted by T cells was investigated. It was found that both GAT-specific and TGAL-specific helper factors from CBA/H mice (H-2k) were retained and eluted from immunoabsorbents which contained rabbit antibodies directed exclusively against the carbohydrate-defined Ia antigens (anti-Iak). Mouse anti-Iak immunoabsorbents which carried antibodies against both the carbohydrate-defined and protein-defined Ia antigens also removed both helper factors. However, these immunoabsorbents were unable to bind helper factor when they were pretreated with a dialyzable material in normal (H-2k) mouse serum which is known to neutralise antibodies against the carbohydrate-defined Ia antigens.