The skin-reactive factor (SRF) synthesized by immunized guinea pig lymphocytes cultured in the presence of antigen or mitogen has been further investigated taking advantage of the vascular permeability induced by culture supernatant in the skin of the same species. The extravasation of Evans blue between the 20th minute and the 4th hour following the intradermal injection of culture supernatants reflects SRF activity, with precision. By means of this test lymph node lymphocytes in living condition are shown to produce a purer form of SRF than lymphocytes from other sources. Pharmacological studies disclosed that certain plasma fractions had an enhancing effect. Certain inhibitors of the kinins and of the coagulation systems had a depressing effect on SRF activity. The substance, with a suggested molecular weight of 39,000, could not be identified with any of the common mediators of inflammation. Several other properties of lymphocyte culture supernatants have been studied for their relationship on vascular permeability.