Four clones that encode defensins, a group of microbicidal and cytotoxic peptides made by neutrophils, were isolated from an HL-60 human promyelocytic leukemia cDNA library. Analysis of these clones indicated that the defensins are made as precursor proteins, which must be cleaved to yield the mature peptides. Defensin mRNA was detected in normal bone marrow cells, but not in normal peripheral blood leukocytes. Defensin transcripts were also found in the peripheral leukocytes of some leukemia patients and in some lung and intestine tissues. Defensin mRNA content was augmented by treatment of HL-60 cells with dimethyl sulfoxide. These results define important aspects of the mechanism of synthesis and the tissue-specific expression of a major group of neutrophil granule proteins.