We identified antibacterial components in human T and natural killer (NK) cells by using freshly isolated lymphocytes enriched for T and NK cells as starting material. After growing these lymphocytes for 5 days in the presence of interleukin (IL)–2, we isolated and characterized several antibacterial peptides/proteins from the supernatant—α-defensins (HNP 1-3), LL-37, lysozyme, and a fragment of histone H2B—although other active components were also present. We then used reverse transcriptase–polymerase chain reaction to search for expression of the gene coding for LL-37 in several B-cell lines, γδ T-cell lines, NK clones, and one monocytic cell line, with positive results, but found no expression in several αβ T-cell lines. The α-defensins (HNP 1-3) were also found to be expressed in several of these cell lines. To confirm the presence of these antibacterial peptides in lymphocytes, we localized them to NK, γδ T cells, B cells, and monocytes/macrophages by using double-staining immunohistochemical analysis of freshly isolated lymphocytes. We also found that primary cultures of lymphocytes transcribe and secrete LL-37 and that these processes are affected by IL-6 and interferon-γ. In addition, we demonstrated that LL-37 has chemotactic activity for polymorphonuclear leukocytes and CD4 T lymphocytes, whereas others have shown chemotactic activity for human α-defensins (HNP 1-2). These findings suggest that microbicidal peptides are effector molecules of lymphocytes and that antibacterial activity previously shown to be derived from T and NK cells may be partly mediated by the antibacterial peptides LL-37 and HNP 1-3.