Human interleukin 4 is a 129 amino acid lymphokine secreted by activated T cells that exerts pleiotropic biological effects on B and T lymphocytes and other hematopoietic cells. Structure-function relations were studied by employing selective proteolytic cleavage of purified recombinant human interleukin 4 (rhuIL-4). Limited proteolysis with endoprotease Glu-C from Staphylococcus aureus (V8) produced two digestion products that were observed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with apparent molecular weight values of 19K (I) and 15K (II), respectively. These species were isolated by reversed-phase HPLC. Amino acid sequencing indicated that species II was an 84 amino acid core fragment extending from Gln-20 to Glu-103 and containing a hydrolyzed peptide bond at Glu-26. On the basis of known disulfide bond assignments, it was concluded that species II was stabilized by two disulfide bonds (Cys-24/Cys-65 and Cys-46/Cys-99). Analysis of its secondary structure by circular dichroism revealed a high content of alpha helix. Species I was the full-length rhuIL-4 with selective cleavage at Glu-26 and Glu-103. Both species I and II were inactive in an in vitro assay based on proliferation of peripheral blood lymphocyte blasts and lacked the ability to bind to teh rhuIL-4 receptor on Daudi cells. In order to elucidate further the role of the residues removed by S. aureus V8 protease, rabbit antisera were raised to synthetic peptides corresponding to residues 1-26 at the N-terminus and 104-129 at the C-terminus. Only antisera directed to the C-terminal peptide inhibited binding of 125I-rhuIL-4 to Daudi cells.(ABSTRACT TRUNCATED AT 250 WORDS)