This report describes an activity in serum from mice that were contact‐sensitized with picryl chloride (PCl) 1 to 4 days earlier. Immune serum, when given i.v., transfers the ability to elicit an immediate hypersensitivity‐like ear swelling reaction in naive recipients following local challenge with PCl. This serum activity is due to an antigen‐ binding T cell factor that shares some properties with IgE antibody. The activity is antigen specific, and due to an antigen‐binding moiety that is heat labile (56°C, 4 h). However, unlike IgE antibody the serum activity is resistant to reduction and alkylation, and is retained by columns of Sepharose beads coupled with polyclonal or monoclonal antibodies that react with antigen‐specific T cell factors from other systems. These columns did not retain IgE antibody activity in our experiments. Importantly, the serum activity was not retained by columns linked with antibodies directed to mouse immunoglobulins, which do retain IgE activity. We conclude from these data that the activity in PCl immune serum is not caused by IgE antibody, and is due to the presence of the previously described antigen‐specific T cell factor (PCl‐factor), that can activate serotonin‐containing cells, such as mast cells, to release the vasoactive amine serotonin. PCl‐factor transfers the ability to elicit an immediate hypersensitivity‐like reaction that is an early component of delayed‐type hypersensitivity. The presence of this T cell factor in the serum of actively sensitized mice provides a means to sensitize tissues throughout the body for this required, initial, serotonin‐dependent component of delayed‐type hypersensitivity reactions.