To understand the regulation of the production of antimicrobial cationic peptide (CP) in leukocytes, expression of the CP gene was evaluated in various types of leukocytes using guinea pig neutrophils, monocytes, macrophages, eosinophils, lymphocytes and bone marrow cells. Acid-urea PAGE and SDS-PAGE/immunoblot analyses showed that CP was present in neutrophils and bone marrow cells, but not in other leukocytes. Northern blot and transcription run-off analyses revealed that only bone marrow cells expressed CP mRNA and transcribed the CP gene. Interestingly, in situ hybridization analysis using bone marrow cells demonstrated that CP mRNA was expressed in the neutrophil precursor cells, such as promyelocytes and myelocytes, but was not detected in the mature neutrophils and other bone marrow cells. Moreover, immunocytochemical study indicated that CP was present in the neutrophil precursor cells and the mature neutrophils in the bone marrow. Thus, the CP gene appears to be expressed during a limited period of neutrophil maturation, and CP is likely synthesized by the neutrophil precursor cells in the bone marrow.