We have isolated a 725-base-pair cDNA clone for human eosinophil-derived neurotoxin (EDN). EDN is a distinct cationic protein of the eosinophil's large specific granule known primarily for its ability to induce ataxia, paralysis, and central nervous system cellular degeneration in experimental animals (Gordon phenomenon). The open reading frame encodes a 134-amino acid mature polypeptide with a molecular mass of 15.5 kDa and a 27-residue amino-terminal hydrophobic leader sequence. The sequence of the mature polypeptide is identical to that reported for human urinary ribonuclease [Beintema, J. J., Hofsteenge, J., Iwama, M., Morita, T., Ohgi, K., Irie, M., Sugiyama, R. H., Schieven, G. L., Dekker, C. A. & Glitz, D. G. (1988) Biochemistry 27, 4530-4538] and to the amino-terminal sequence of human liver ribonuclease [Sorrentino, S., Tucker, G. K. & Glitz, D. G. (1988) J. Biol. Chem. 263, 16125-16131]; the cDNA encodes a tryptophan in position 7, which was previously unidentified in the amino acid sequences of EDN or the urinary and liver ribonucleases. Both EDN and the related granule protein, eosinophil cationic protein, have ribonucleolytic activity; sequence similarities among EDN, eosinophil cationic protein, ribonucleases from liver, urine, and pancreas, and angiogenin define a ribonuclease multigene family. mRNA encoding EDN was detected in uninduced HL-60 cells and was up-regulated in cells induced toward eosinophilic differentiation with B-cell growth factor 2/interleukin 5 and toward neutrophilic differentiation with dimethyl sulfoxide. EDN mRNA was detected in mature neutrophils even though EDN-like neurotoxic activity is not found in neutrophil extracts. These results suggest that neutrophils contain a protein that is closely related or identical to EDN.