Human neutrophil peptides (HNPs) 1,2 and 3, also called defensins, have high antimicrobial activity and are cytotoxic to both normal and transformed human cells. The localization of HNPs 1-3 and their mRNAs in cells of normal human bone marrow and peripheral blood was investigated. Immunoreactivity for HNPs 1-3 appeared in the promyelocyte stage and increased with the maturation of the neutrophil series. HNPs 1-3 were absent from such mononuclear phagocyte systems as peripheral monocytes, alveolar macrophages, and Kupffer cells in the liver. HNP 1-3 mRNAs were detected in bone marrow cells by Northern blot analysis and in peripheral leukocytes by the reverse transcription-polymerase chain reaction (RT-PCR). HNPs 1-3 are restricted to cells of neutrophil lineage and seem to function as microbicidal agents in the neutrophil-mediated defense system.