Recent evidence indicates that interleukin 1 (IL-1) from different sources has varying molecular weights, amino acid and gene sequences and biological properties. In previous experiments, it has been shown that monocyte derived IL-1 was chemotactic for lymphocytes and stimulated their binding to endothelial cells (EC). These phenomena are important in the emigration of lymphocytes in inflammatory states. In the present investigation, EC were stimulated with LPS and from the supernatants the IL-1 activity was isolated. After AcA 54 gel filtration, the active 17 kD fraction was further purified by chromatofocusing, yielding active fractions with pI of 7.0 and 5.0. All of these fractions showed T lymphocyte chemotactic activity, stimulated the binding of T cells to EC and the proliferation of fibroblasts. It is concluded, therefore, that EC-derived IL-1 has similar biological activity to that of monocyte-derived IL-1 and that it can exert a true autocrine effect at the blood-tissue endothelial interface.