When cultured in the presence of specific antigen, lymphocytes from delayed hypersensitive guinea pigs release a precursor substance (ECFp) which can be activated by specific immune complexes to yield an eosinophil chemotactic factor (ECF). The generation of ECFp like that of migration inhibition factor (MIF) is carrier specific. The immune complexes which activate ECFp must contain IgG2 antibody with specificity directed against the antigen which was used to stimulate the lymphocytes to produce ECFp. Previous studies have shown that this reaction does not involve complement. It was not possible to dissociate MIF activity from ECFp activity by a combination of sephadex chromotography and agarose block electrophoresis. However, columns containing sepharose beads conjugated with specific antibody removed ECFp but not MIF from the supernatant fluids. Antigen-conjugated columns did not affect either activity. If the supernatants were activated with immune complexes before passage through the columns, then eosinophil activity could not be removed by either column. These results all suggest that ECFp co-precipitates with immune complexes by virtue of an antigenic fragment it contains. The activation of ECFp to ECF involves loss of this fragment.